avcodec/pthread_frame: fix missing unlock on error

Fixes CID1197057
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/msrle: check return code for success before use

The check is possibly redundant, but better to check for errors
that dont occur than to skip the check and crash

Fixes CID1197060

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>