avcodec/aac_defines: Add missing () to AAC_HALF_SUM() macro

Fixes: runtime error: shift exponent 1073741848 is too large for 32-bit type 'INTFLOAT' (aka 'int')
Fixes: 1880/clusterfuzz-testcase-minimized-4900645322620928

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/aac_defines.h

@@ -72,7 +72,7 @@ typedef int                 AAC_SIGNE;
 #define AAC_MSUB31_V3(x, y, z)    (int)((((int64_t)(x) * (z)) - \
                                       ((int64_t)(y) * (z)) + \
                                         0x40000000) >> 31)
-#define AAC_HALF_SUM(x, y)  (x) >> 1 + (y) >> 1
+#define AAC_HALF_SUM(x, y)  (((x) >> 1) + ((y) >> 1))
 #define AAC_SRA_R(x, y)     (int)(((x) + (1 << ((y) - 1))) >> (y))
 
 #else