don't use a null display

Change-Id: Iab0a53abd723f0309f40742636315079a4b2c532

vcl/source/opengl/OpenGLContext.cxx

@@ -491,13 +491,18 @@ private:
 public:
     TempErrorHandler(Display* dpy, errorHandler newErrorHandler):
         mdpy(dpy)
+    {
+        if (mdpy)
         {
             XLockDisplay(dpy);
             XSync(dpy, false);
             oldErrorHandler = XSetErrorHandler(newErrorHandler);
         }
+    }
 
     ~TempErrorHandler()
+    {
+        if (mdpy)
         {
             // sync so that we possibly get an XError
             glXWaitGL();
@@ -505,6 +510,7 @@ public:
             XSetErrorHandler(oldErrorHandler);
             XUnlockDisplay(mdpy);
         }
+    }
 };
 
 static bool errorTriggered;