Return 0 explicitly for OD_ILOG(0).

Change-Id: I9fd4052f0b3f280496882f4475c97b1b5a75934f

vp10/common/odintrin.h

@@ -31,6 +31,6 @@ typedef int16_t dering_in;
    documentation (and that of the x86 BSR instruction that implements it), so
    we have to special-case it.
   We define a special version of the macro to use when x can be zero.*/
-# define OD_ILOG(x) (OD_ILOG_NZ(x) & -!!(x))
+# define OD_ILOG(x) ((x) ? OD_ILOG_NZ(x) : 0)
 
 #endif