fixing mem corruption

Originally committed as revision 1729 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.c

@@ -364,6 +364,8 @@ static void conv411(uint8_t *dst, int dst_wrap,
     int w, c;
     uint8_t *s1, *s2, *d;
 
+    width>>=1;
+
     for(;height > 0; height--) {
         s1 = src;
         s2 = src + src_wrap;