ffplay: Separate AVFrame from AVSubtitle

This is more robust and should avoid false positives in coverity
That is if the type is ever mixed up, after this patch that would
just be a null pointer dereference instead of a random possibly
out of array access

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
