First, make s signed, so that comparisons against end - p will not be made as
unsigned, making the check incorrectly pass if p is beyond end.
Also ensure that p will never be > end, so the code is correct also if
buf is not padded.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20014 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b