Ensure the UTF-8 string for a xcb_keycode_t has the correct length.

On my platform (Archlinux, libxcb 1.11-1) and a recent build of Qt dev
I noticed that writing anything to a QLineEdit was broken, as not only
the character for the pressed key was inserted, but also what looks
like whitespace. A simple debug program showed that a "\u0000" is
appended to every character in the QString text associated with a
QKeyEvent, which brought me to QXcbKeyboard::lookupString.

The QByteArray to QString conversion using QString::fromUtf8 includes
the \x00 byte at the end of the buffer.

By leveraging the size returned by the xcb API and passing that to
QString::fromUtf8 we can prevent this problem from arising.

Change-Id: Ic1d4390e4154e9ed729cd23286811d6eecdf54f6
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/plugins/platforms/xcb/qxcbkeyboard.cpp

@@ -1518,8 +1518,8 @@ QString QXcbKeyboard::lookupString(struct xkb_state *state, xcb_keycode_t code)
     QByteArray chars;
     chars.resize(1 + xkb_state_key_get_utf8(state, code, 0, 0));
     // equivalent of XLookupString
-    xkb_state_key_get_utf8(state, code, chars.data(), chars.size());
-    return QString::fromUtf8(chars);
+    const int size = xkb_state_key_get_utf8(state, code, chars.data(), chars.size());
+    return QString::fromUtf8(chars.constData(), size);
 }
 
 void QXcbKeyboard::handleKeyPressEvent(const xcb_key_press_event_t *event)