basesrc: avoid potential deadlock

basesrc: avoid potential deadlock

In gst_base_src_start_complete() we do a perform_seek() that will eventually
start the streaming thread which acquires the live lock and then goes to sleep
in the case of appsrc. Right after we perform seek we also try to acquire the
live lock which might then deadlock.

fix this by taking the stream lock before performing the seek. This makes sure
that the streaming thread cannot start and grab the live lock until we are done
and release the stream lock again.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=676048