avfilter/vf_psnr: Prevent integer overflow.

The 32-bit integer accumulator in MSE computation can overflow for 8-bit frame data.
(e.g., for 1080p white frame compared to a black frame can give sum of 255*255*1080*1920 > 2^32).

Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
