ffmpeg: fix deadlock regression in threading error handing

Commit fc9c857c introduced deadlock regression when processing too many inputs:
  ffmpeg $(seq -f " -f lavfi -i aevalsrc=0:d=%.0f" 70) -vf concat=n=70:v=0:a=1 -f null -
Happens for different number of inputs, depending on available memory size,
overcommit settings, ulimits, etc. Easily noticeable for 32-bit builds,
that exhaust address space allocating 8-10 MB stack for each thread.
Earlier ffmpeg versions exited with unhelpful "Conversion failed!" message.

This patch fixes both problems: it frees the queue to prevent deadlock
and adds a meaningful error message if pthread_create() fails.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>