avcodec/internal: Add () to argument of FF_SIGNBIT() to ensure correct order or operations

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

libavcodec/internal.h

@@ -35,7 +35,7 @@
 
 #define FF_SANE_NB_CHANNELS 63U
 
-#define FF_SIGNBIT(x) (x >> CHAR_BIT * sizeof(x) - 1)
+#define FF_SIGNBIT(x) ((x) >> CHAR_BIT * sizeof(x) - 1)
 
 #if HAVE_AVX
 #   define STRIDE_ALIGN 32