Free the read buffer.

==26538== 4,096 bytes in 1 blocks are definitely lost in loss record 781 of 781
==26538==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==26538==    by 0x4005C66: realloc (vg_replace_malloc.c:476)
==26538==    by 0x405244D: g_realloc (gmem.c:181)
==26538==    by 0x420E066: _g_dbus_worker_do_read_unlocked (gdbusprivate.c:780)
==26538==    by 0x420E1D1: _g_dbus_worker_do_read (gdbusprivate.c:812)
==26538==    by 0x420F14A: _g_dbus_worker_thread_begin_func (gdbusprivate.c:1318)
==26538==    by 0x420D2ED: invoke_caller (gdbusprivate.c:266)
==26538==    by 0x404DA7C: g_idle_dispatch (gmain.c:4224)
==26538==    by 0x4049FCD: g_main_dispatch (gmain.c:2119)
==26538==    by 0x404B2C1: g_main_context_dispatch (gmain.c:2672)
==26538==    by 0x404B716: g_main_context_iterate (gmain.c:2750)
==26538==    by 0x404BE7F: g_main_loop_run (gmain.c:2958)
==26538==    by 0x420D2B5: shared_thread_func (gdbusprivate.c:248)
==26538==    by 0x4077958: g_thread_create_proxy (gthread.c:1897)
==26538==    by 0x57D918: start_thread (pthread_create.c:301)
==26538==    by 0x4C6CBD: clone (clone.S:133)

Bug #627187.