ffmpeg: use parse_number_and_die() when it makes sense

Prefer parse_number_or_die() over atoi()/atol() parsing for the options:
-pass, -top, -vc, and -qscale.

Improve input validation.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>