lavu/eval: handle div by zero in mod().

Similarly to 03f5043f5, we use the FTRAPV config setting.
lavu/base64: extend/clarify doxy for the base64 API

Also improve overall consistency.
doc/faq: add -start_number example

Also add example showing cat piping images to ffmpeg.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
av_rescale: support passing MIN/MAX through

Reviewed-by: Clément Bœsch <ubitux@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat_seek_file: Dont attempt to rescale INT64_MIN/MAX

This fixes a integer overflow in fate

Reviewed-by: Clément Bœsch <ubitux@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>