Use safe way to get at a vector's data pointer

&std::vector<C>[0] is not guaranteed to not throw if the
vector is empty. Better safe than sorry.

binding-mruby/mrb-ext/rwmem.cpp

@@ -101,7 +101,7 @@ int RWMemGetData(SDL_RWops *ops, void *buffer)
 	ByteVec *v = getRWPrivate(ops);
 
 	if (buffer)
-		memcpy(buffer, &(*v)[0], v->size());
+		memcpy(buffer, dataPtr(*v), v->size());
 
 	return v->size();
 }