a2dp: Fix crash when SEP codec has not been initialized

a2dp: Fix crash when SEP codec has not been initialized

If SEP has not been properly discovered avdtp_get_codec may return NULL
thus causing crashes such as when running AVRCP/TG/VLH/BI-01-C after
AVRCP/TG/RCR/BV-04-C.

Prevent remote endpoint registration if its codec is not available.

Remove queue_isempty check from store_remote_seps since that prevents
cleaning up if no seps could be registered.

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>