With a failed SASL connection in Pidgin3 I got following leaks in valgrind:
==603383== 160 (64 direct, 96 indirect) bytes in 1 blocks are definitely lost in loss record 26,955 of 32,695
==603383==    at 0x4D6859F: g_type_create_instance (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x4D4B86F: ??? (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x4D4E322: g_object_new_valist (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x4D4E67C: g_object_new (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x5D591FE: birb_queued_output_stream_new (birbqueuedoutputstream.c:150)
==603383==    by 0xFE3039D: ibis_client_start (ibisclient.c:794)
==603383==    by 0xFE2EC1A: ibis_client_connect_cb (ibisclient.c:253)
==603383==    by 0x588A6E2: ??? (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.8000.2)
...
==603383== 4,256 (64 direct, 4,192 indirect) bytes in 1 blocks are definitely lost in loss record 32,060 of 32,695
==603383==    at 0x4D6859F: g_type_create_instance (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x4D4B86F: ??? (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x4D4E322: g_object_new_valist (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0x4D4E67C: g_object_new (in /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0.8000.2)
==603383==    by 0xFE303D4: ibis_client_start (ibisclient.c:798)
==603383==    by 0xFE2EC1A: ibis_client_connect_cb (ibisclient.c:253)
==603383==    by 0x588A6E2: ??? (in /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0.8000.2)

I'm not sure why these leaks don't show up with a successful connection but I checked valgrind with this patch and a successful connection and there were no double frees or invalid reads/writes.

These should probably be in ibis_client_disconnect as the stream objects should only be valid while a connection is established.
Can you put them there and run your test again to verify that that fixes it?

Great work, thanks!!