motion_est: Limit motion vector search range to MAX_MV

Fixes out of array reads with videos exceeding MAX_MV

Found-by: Thierry Foucu
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/motion_est.c

@@ -516,6 +516,7 @@ static inline void get_limits(MpegEncContext *s, int x, int y)
 {
     MotionEstContext * const c= &s->me;
     int range= c->avctx->me_range >> (1 + !!(c->flags&FLAG_QPEL));
+    int max_range = MAX_MV >> (1 + !!(c->flags&FLAG_QPEL));
 /*
     if(c->avctx->me_range) c->range= c->avctx->me_range >> 1;
     else                   c->range= 16;
@@ -537,6 +538,8 @@ static inline void get_limits(MpegEncContext *s, int x, int y)
         c->xmax = - x + s->mb_width *16 - 16;
         c->ymax = - y + s->mb_height*16 - 16;
     }
+    if(!range || range > max_range)
+        range = max_range;
     if(range){
         c->xmin = FFMAX(c->xmin,-range);
         c->xmax = FFMIN(c->xmax, range);