mpegvideo: fix out of heap array accesses

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

libavcodec/mpegvideo.c

@@ -1436,7 +1436,7 @@ static void draw_line(uint8_t *buf, int sx, int sy, int ex, int ey,
             y  = (x * f) >> 16;
             fr = (x * f) & 0xFFFF;
             buf[y * stride + x]       += (color * (0x10000 - fr)) >> 16;
-            buf[(y + 1) * stride + x] += (color *            fr ) >> 16;
+            if(fr) buf[(y + 1) * stride + x] += (color *            fr ) >> 16;
         }
     } else {
         if (sy > ey) {
@@ -1453,7 +1453,7 @@ static void draw_line(uint8_t *buf, int sx, int sy, int ex, int ey,
             x  = (y*f) >> 16;
             fr = (y*f) & 0xFFFF;
             buf[y * stride + x]     += (color * (0x10000 - fr)) >> 16;
-            buf[y * stride + x + 1] += (color *            fr ) >> 16;
+            if(fr) buf[y * stride + x + 1] += (color *            fr ) >> 16;
         }
     }
 }