According to valgrind:

16 bytes in 2 blocks are definitely lost in loss record 1,408 of 3,437
   at 0x4841B84: calloc (vg_replace_malloc.c:623)
   by 0x51620C3: g_malloc0 (gmem.c:134)
   by 0x1F9CB: mms_transfer_dbus_handle_enable_updates (mms_transfer_dbus.c:246)

if the corresponding conversation is open in Messages UI when MMS is being
sent or received.