Catch more possible instances of passing NULL to memcpy.

Actually fixes #1976.

src/libutil/hash.cc

@@ -191,6 +191,7 @@ Hash::Hash(const std::string & s, HashType type)
         auto d = base64Decode(std::string(s, pos));
         if (d.size() != hashSize)
             throw BadHash("invalid base-64 hash '%s'", s);
+        assert(hashSize);
         memcpy(hash, d.data(), hashSize);
     }