Fix crash when reading mtllib command

The code was assuming the mtllib command to be near the beginning of the file, and therefore to always be read by the first thread.

experimental/tinyobj_loader_opt.h

@@ -1414,8 +1414,9 @@ bool parseObj(attrib_t *attrib, std::vector<shape_t> *shapes,
             }
 
             if (command.type == COMMAND_MTLLIB) {
+              // Save the indices of the `mtllib` command in `commands` to easily find it later
               mtllib_t_index = t;
-              mtllib_i_index = commands->size();
+              mtllib_i_index = commands[t].size();
             }
 
             commands[t].emplace_back(std::move(command));