GHOST/X11: Quiet warning about key-release events having their utf8 set

Quiet warning from [0], no functional change as the this information
was always ignored.

Key release events shouldn't have associated text, this was cleared
for wmEvent's, so there is no reason to pass it from GHOST.

[0]: d6fef73ef1

intern/ghost/intern/GHOST_SystemX11.cpp

@@ -1194,6 +1194,11 @@ void GHOST_SystemX11::processEvent(XEvent *xe)
       }
 #endif
 
+      if (type != GHOST_kEventKeyDown) {
+        ascii = 0;
+        utf8_buf = nullptr;
+      }
+
       g_event = new GHOST_EventKey(
           getMilliSeconds(), type, window, gkey, ascii, utf8_buf, is_repeat);