Fixes a leak for images in requests:
3,627 bytes in 1 blocks are definitely lost in loss record 35,636 of 36,499
   at 0x484386F: malloc (vg_replace_malloc.c:393)
   by 0x4980168: g_malloc (gmem.c:130)
   by 0x4995723: UnknownInlinedFun (gstrfuncs.c:426)
   by 0x4995723: g_memdup2 (gstrfuncs.c:419)
   by 0x4D3E534: purple_request_field_image_new (request.c:1677)
   by 0x1CA970B1: purple_demo_protocol_request_fields_activate (purpledemoprotocolactions.c:553)
   by 0x4DB5FBF: g_closure_invoke (gclosure.c:832)
   by 0x4DE3D85: signal_emit_unlocked_R.isra.0 (gsignal.c:3796)
   by 0x4DD3419: g_signal_emit_valist (gsignal.c:3549)
   by 0x4DD3632: g_signal_emit (gsignal.c:3606)
   by 0x581C784: g_simple_action_activate (gsimpleaction.c:227)
   by 0x581B132: g_action_activate (gaction.c:399)
   by 0x51221B6: UnknownInlinedFun (gtkmenutrackeritem.c:842)
   by 0x51221B6: gtk_popover_item_activate.lto_priv.0 (gtkmenusectionbox.c:200)

and leak of the GObject wrapper for request fields:
4 bytes in 1 blocks are possibly lost in loss record 713 of 36,499
   at 0x484386F: malloc (vg_replace_malloc.c:393)
   by 0x4980168: g_malloc (gmem.c:130)
   by 0x4995602: g_strdup (gstrfuncs.c:363)
   by 0x488B324: create_list_field (gtkrequest.c:1504)
   by 0x488D1D7: pidgin_request_fields (gtkrequest.c:2160)
   by 0x4D3FFB8: purple_request_fields (request.c:2283)
   by 0x1CA971D6: purple_demo_protocol_request_fields_activate (purpledemoprotocolactions.c:562)
   by 0x4DB5FBF: g_closure_invoke (gclosure.c:832)
   by 0x4DE3D85: signal_emit_unlocked_R.isra.0 (gsignal.c:3796)
   by 0x4DD3419: g_signal_emit_valist (gsignal.c:3549)
   by 0x4DD3632: g_signal_emit (gsignal.c:3606)
   by 0x581C784: g_simple_action_activate (gsimpleaction.c:227)
56 bytes in 1 blocks are possibly lost in loss record 19,209 of 36,499
   at 0x48486AF: realloc (vg_replace_malloc.c:1451)
   by 0x498071F: g_realloc (gmem.c:201)
   by 0x49592C4: g_data_set_internal (gdataset.c:474)
   by 0x488B347: create_list_field (gtkrequest.c:1504)
   by 0x488D1D7: pidgin_request_fields (gtkrequest.c:2160)
   by 0x4D3FFB8: purple_request_fields (request.c:2283)
   by 0x1CA971D6: purple_demo_protocol_request_fields_activate (purpledemoprotocolactions.c:562)
   by 0x4DB5FBF: g_closure_invoke (gclosure.c:832)
   by 0x4DE3D85: signal_emit_unlocked_R.isra.0 (gsignal.c:3796)
   by 0x4DD3419: g_signal_emit_valist (gsignal.c:3549)
   by 0x4DD3632: g_signal_emit (gsignal.c:3606)
   by 0x581C784: g_simple_action_activate (gsimpleaction.c:227)
614 (192 direct, 422 indirect) bytes in 8 blocks are definitely lost in loss record 33,122 of 36,499
   at 0x484386F: malloc (vg_replace_malloc.c:393)
   by 0x4980168: g_malloc (gmem.c:130)
   by 0x4997AB5: g_slice_alloc (gslice.c:1074)
   by 0x49980EC: g_slice_alloc0 (gslice.c:1100)
   by 0x4DDF04B: g_type_create_instance (gtype.c:1913)
   by 0x4DC4C1F: g_object_new_internal (gobject.c:2228)
   by 0x4DC6247: g_object_new_with_properties (gobject.c:2391)
   by 0x4DC6FF0: g_object_new (gobject.c:2037)
   by 0x488B2D5: create_list_field (gtkrequest.c:1499)
   by 0x488D1D7: pidgin_request_fields (gtkrequest.c:2160)
   by 0x4D3FFB8: purple_request_fields (request.c:2283)
   by 0x1CA971D6: purple_demo_protocol_request_fields_activate (purpledemoprotocolactions.c:562)

and a request label:
128 bytes in 1 blocks are definitely lost in loss record 27,735 of 36,499
   at 0x484378A: malloc (vg_replace_malloc.c:392)
   by 0x484870B: realloc (vg_replace_malloc.c:1451)
   by 0x498071F: g_realloc (gmem.c:201)
   by 0x499A343: g_string_maybe_expand (gstring.c:92)
   by 0x499A3BF: g_string_sized_new (gstring.c:116)
   by 0x4981BCB: g_markup_escape_text (gmarkup.c:2323)
   by 0x4887D8A: pidgin_request_escape (gtkrequest.c:360)
   by 0x488CED9: pidgin_request_fields (gtkrequest.c:2105)
   by 0x4D3FFB8: purple_request_fields (request.c:2283)
   by 0x1CA971D6: purple_demo_protocol_request_fields_activate (purpledemoprotocolactions.c:562)
   by 0x4DB5FBF: g_closure_invoke (gclosure.c:832)
   by 0x4DE3D85: signal_emit_unlocked_R.isra.0 (gsignal.c:3796)

and one in the demo protocol:
40 bytes in 1 blocks are definitely lost in loss record 15,277 of 36,499
   at 0x484386F: malloc (vg_replace_malloc.c:393)
   by 0x4AE3017: __vasprintf_internal (vasprintf.c:71)
   by 0x49C2E81: UnknownInlinedFun (stdio2.h:169)
   by 0x49C2E81: g_vasprintf (gprintf.c:340)
   by 0x4998EA0: g_strdup_vprintf (gstrfuncs.c:553)
   by 0x4998F5C: g_strdup_printf (gstrfuncs.c:583)
   by 0x1CA9584E: purple_demo_protocol_request_input_activate (purpledemoprotocolactions.c:203)
   by 0x4DB5FBF: g_closure_invoke (gclosure.c:832)
   by 0x4DE3D85: signal_emit_unlocked_R.isra.0 (gsignal.c:3796)
   by 0x4DD3419: g_signal_emit_valist (gsignal.c:3549)
   by 0x4DD3632: g_signal_emit (gsignal.c:3606)
   by 0x581C784: g_simple_action_activate (gsimpleaction.c:227)
   by 0x581B132: g_action_activate (gaction.c:399)

Ran valgrind and confirmed above leaks were not there after opening Request Input and Request Fields.