media-playback: Fix possible crash if frame width or height is zero

If a frame has a width or height of zero, this value will make it into
libobs/media-io/video-frame.c:video_frame_init and cause linesizes or
heights to be zero, which will result in a bmalloc(0) call and OBS will
crash.

Instead of letting the call stack get that far, check the frame width
and height here at the source, log an error, and return early if the
frame width or height are zero.