Use std::string instead of const char* for GetKey function

tiny_gltf.h

@@ -3000,7 +3000,9 @@ json_const_iterator ObjectEnd(const json &o) {
 #endif
 }
 
-const char *GetKey(json_const_iterator &it) {
+// Making this a const char* results in a pointer to a temporary when
+// TINYGLTF_USE_RAPIDJSON is off.
+std::string GetKey(json_const_iterator &it) {
 #ifdef TINYGLTF_USE_RAPIDJSON
   return it->name.GetString();
 #else