As I used simple RGBA formats for subtitles and for the video texture if
avfilter is disabled I kind of assumed that sws_scale won't access data
pointers and strides above index 0, but apparently that is not the case.

Fixes Coverity CID 1396737, 1396738, 1396739, 1396740.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Marton Balint <cus@passwd.hu>