avcodec/error_resilience: comment out unused  assignment

Its not removed as the code which uses this is out-commented too.

Found-by: CSA
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/error_resilience: make error an local variable where possible

This makes the code easier to understand as the scope of the variable is
smaller.

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