avcodec/me_cmp: add () to protect avg*s arguments

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

libavcodec/me_cmp.c

@@ -108,8 +108,8 @@ static int sum_abs_dctelem_c(int16_t *block)
     return sum;
 }
 
-#define avg2(a, b) ((a + b + 1) >> 1)
-#define avg4(a, b, c, d) ((a + b + c + d + 2) >> 2)
+#define avg2(a, b) (((a) + (b) + 1) >> 1)
+#define avg4(a, b, c, d) (((a) + (b) + (c) + (d) + 2) >> 2)
 
 static inline int pix_abs16_c(MpegEncContext *v, uint8_t *pix1, uint8_t *pix2,
                               ptrdiff_t stride, int h)