X11: Fix memory leak in handle_key_event

Extracted from #27189.

(cherry picked from commit d191ffb47ddd50df470ef1714c7df6b4da9c7bca)

platform/x11/os_x11.cpp

@@ -1665,6 +1665,7 @@ void OS_X11::handle_key_event(XKeyEvent *p_event, bool p_echo) {
 
 				input->accumulate_input_event(k);
 			}
+			memfree(utf8string);
 			return;
 		}
 		memfree(utf8string);