lavf: Make sure avg_frame_rate can be calculated without integer overflow

If either of the deltas is too large for the multiplications to
succeed, don't use this for setting the avg frame rate.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
