avcodec/jpeglsdec: assert that overflows end at exactly x=w

If that ever was untrue, there would be a bug in the code
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeglsdec.c

@@ -280,6 +280,7 @@ static inline void ls_decode_line(JLSState *state, MJpegDecodeContext *s,
 
             if (x >= w) {
                 av_log(NULL, AV_LOG_ERROR, "run overflow\n");
+                av_assert0(x <= w);
                 return;
             }