fix bug for negative shift value

Reviewed By: luciang

Differential Revision: D3407128

fbshipit-source-id: 2c00387d6e068f9e052f539198ae2f985d265c74

folly/Hash.h

@@ -281,7 +281,7 @@ inline uint64_t fnv64(const std::string& str,
 
 inline uint32_t hsieh_hash32_buf(const void* buf, size_t len) {
   // forcing signed char, since other platforms can use unsigned
-  const signed char* s = reinterpret_cast<const signed char*>(buf);
+  const unsigned char* s = reinterpret_cast<const unsigned char*>(buf);
   uint32_t hash = static_cast<uint32_t>(len);
   uint32_t tmp;
   size_t rem;