4xm: fix calculation of the next output line position in decode_i2_frame().

The current code doesn't work unless width is an exact multiple of 16.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/4xm.c

@@ -640,7 +640,7 @@ static int decode_i2_frame(FourXContext *f, const uint8_t *buf, int length){
             }
             dst+=16;
         }
-        dst += 16*stride - width;
+        dst += 16 * stride - x;
     }
 
     return 0;