VP8LColorCacheCopy: promote an int before shifting

quiets an msvs /analyze warning, the cast is due to an earlier msvs
build warning.
C6297: Arithmetic overflow:  32-bit value is shifted, then cast to
64-bit value.  Results might not be an expected value.

Change-Id: I0bb6cda57879f2fbd1e3515f6753a11bc08d14ac

src/utils/color_cache.c

@@ -45,5 +45,5 @@ void VP8LColorCacheCopy(const VP8LColorCache* const src,
   assert(dst != NULL);
   assert(src->hash_bits_ == dst->hash_bits_);
   memcpy(dst->colors_, src->colors_,
-         (size_t)(1 << dst->hash_bits_) * sizeof(*dst->colors_));
+         ((size_t)1u << dst->hash_bits_) * sizeof(*dst->colors_));
 }