don't use a null display

Change-Id: Iab0a53abd723f0309f40742636315079a4b2c532
(cherry picked from commit c94d60d6)
Signed-off-by: Michael Stahl <mstahl@redhat.com>

vcl/source/opengl/OpenGLContext.cxx

@@ -465,18 +465,24 @@ public:
     TempErrorHandler(Display* dpy, errorHandler newErrorHandler):
         mdpy(dpy)
     {
-        XLockDisplay(dpy);
-        XSync(dpy, false);
-        oldErrorHandler = XSetErrorHandler(newErrorHandler);
+        if (mdpy)
+        {
+            XLockDisplay(dpy);
+            XSync(dpy, false);
+            oldErrorHandler = XSetErrorHandler(newErrorHandler);
+        }
     }
 
     ~TempErrorHandler()
     {
-        // sync so that we possibly get an XError
-        glXWaitGL();
-        XSync(mdpy, false);
-        XSetErrorHandler(oldErrorHandler);
-        XUnlockDisplay(mdpy);
+        if (mdpy)
+        {
+            // sync so that we possibly get an XError
+            glXWaitGL();
+            XSync(mdpy, false);
+            XSetErrorHandler(oldErrorHandler);
+            XUnlockDisplay(mdpy);
+        }
     }
 };