avcodec/pthread_frame, decode: allow errors to happen on draining

So, all frames and errors are correctly reported in order.
Also limit the numbers of error during draining to prevent infinite loop.

This fix fate failure with THREADS>=4:
  make fate-h264-attachment-631 THREADS=4
This also reverts a755b725ec1d657609c8bd726ce37e7cf193d03f.

Suggested-by: wm4, Ronald S. Bultje, Marton Balint
Reviewed-by: w4 <nfxjfg@googlemail.com>
Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>
