Use full digest length instead of just one byte.

When our universal hash function fell back to MD5 for inputs larger than
supported by H3, the computation only returned the first byte of the MD5 result
instead of as many bytes as needed to cover sizeof(Hasher::digest).

src/probabilistic/Hasher.cc

@@ -108,7 +108,7 @@ Hasher::digest UHF::hash(const void* x, size_t n) const
 
 	MD5(d, 16, d);
 
-	return d[0];
+	return *reinterpret_cast<const Hasher::digest*>(d);
 	}
 
 DefaultHasher::DefaultHasher(size_t k, size_t seed)