X11: correctly handle buffer overflow errors in Xutf8LookupString API call.
X11: correctly handle buffer overflow errors in Xutf8LookupString API call.

We never handled buffer overflow correctly before.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI