Fixes a floating point exception when width and height are not supplied (and therefore are zero).
Allows one to do: ffmpeg -s 1920x1080 -i blah.v210 ... ffmpeg -s 1920x1080 -f v210x -i blah.yuv10 ... Fixes #1869. Reviewed-by: James Almer <jamrial@gmail.com> Reviewed-by: Kieran Kunhya <kierank@obe.tv>