-Werror=logical-op (GCC 6)

...where NoSymbol is apparently defined as zero in X11/X.h (though the X11 docs
at <http://www.x.org/releases/X11R7.7/doc/libX11/libX11/libX11.html> are silent
about that), even though XKeysymToKeycode is specified to return zero, not
NoSymbol, if "the specified KeySym is not defined" (<http://www.x.org/releases/
11R7.7/doc/libX11/libX11/libX11.html#id2813072>); lets assume NoSymbol /is/
zero, and static_assert that assumption

Change-Id: Ib3db59373fb084fcd936e0c7be1b76be994e261f

vcl/unx/generic/app/saldisp.cxx

@@ -737,7 +737,8 @@ OUString SalDisplay::GetKeyNameFromKeySym( KeySym nKeySym ) const
     // return an empty string for keysyms that are not bound to
     // any key code
     KeyCode aKeyCode = XKeysymToKeycode( GetDisplay(), nKeySym );
-    if( aKeyCode != 0 && aKeyCode != NoSymbol )
+    static_assert(NoSymbol == 0, "X11 inconsistency");
+    if( aKeyCode != NoSymbol )
     {
         if( !nKeySym )
             aRet = "???";