fix memory leak originating in purple_prefs_connect_callback

This leak happened just by starting and closing Pidgin. So I started and closed Pidgin and the leak didn't happen anymore. 
Example valgrind output of the leak:
==425973== 21 bytes in 1 blocks are definitely lost in loss record 3,984 of 17,678
==425973==    at 0x4848899: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==425973==    by 0x54CF738: g_malloc (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7200.4)
==425973==    by 0x54E4583: g_strdup (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7200.4)
==425973==    by 0x5796FBE: purple_prefs_connect_callback (prefs.c:1425)
==425973==    by 0x17DF74: setup_menubar (gtkconv.c:3573)
==425973==    by 0x18F25F: pidgin_conv_window_new (gtkconv.c:9477)
==425973==    by 0x18BAAF: pidgin_conversations_init (gtkconv.c:8193)
==425973==    by 0x1BE505: pidgin_ui_init (gtkmain.c:328)
==425973==    by 0x57607F9: purple_core_init (core.c:188)
==425973==    by 0x1BF5C3: main (gtkmain.c:826)