gobex: Fix segfault caused by interrupted transfer

gobex: Fix segfault caused by interrupted transfer

When a obex transfer is interrupted by a peer in the middle, the response
G_OBEX_RSP_FORBIDDEN comes and the transfer is freed in transfer_complete.
However gobex is still ref'ed and gobex->io continues to be writable,
so write_data() and then g_obex_abort() are called. When the abort response
comes, struct obc_transfer is already freed, which leads to the crash.

Backtrace :
__GI___pthread_mutex_lock (mutex=0x65732f74) at pthread_mutex_lock.c:67
0xecc6eeda in dbus_connection_get_object_path_data () from libdbus-1.so.3
0x000457d4 in g_dbus_emit_property_changed_full () at gdbus/object.c:1794
0x00045868 in g_dbus_emit_property_changed () at gdbus/object.c:1832
0x000367f0 in transfer_set_status () at obexd/client/transfer.c:211
0x0003681e in transfer_set_status () at obexd/client/transfer.c:206
xfer_complete () at obexd/client/transfer.c:672
0x00022df6 in transfer_complete () at gobex/gobex-transfer.c:103
0x00022f44 in transfer_abort_response () at gobex/gobex-transfer.c:124
0x00020a0e in handle_response () at gobex/gobex.c:1128
0x00020dde in incoming_data () at gobex/gobex.c:1373

This commit introduces g_obex_drop_tx_queue(), which will be called if
a transfer error detected. After the tx queue is dropped, obex shuts
down gracefully.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>