Plug a mem leak

Plug a mem leak

... and use g_error_matches().

==29535== 1,360 (408 direct, 952 indirect) bytes in 17 blocks are definitely lost in loss record 1,252 of 1,264
==29535==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==29535==    by 0x4057094: g_malloc (gmem.c:134)
==29535==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==29535==    by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==29535==    by 0x403A751: g_error_new_valist (gerror.c:54)
==29535==    by 0x403AAD4: g_set_error (gerror.c:240)
==29535==    by 0x41C06C8: g_socket_send_message (gsocket.c:2967)
==29535==    by 0x421CB64: write_message_continue_writing (gdbusprivate.c:958)
==29535==    by 0x421CE2A: write_message_async (gdbusprivate.c:1049)
==29535==    by 0x421D4DD: maybe_write_next_message (gdbusprivate.c:1291)
==29535==    by 0x421D26B: message_written (gdbusprivate.c:1187)
==29535==    by 0x421D322: write_message_cb (gdbusprivate.c:1216)

Bug #628345.