avoid some OpenGL calls when vcl OpenGL is not enabled

There are even more and it is causing a number of performance issues.
This just fixes the most obvious place.

Change-Id: Id85c13b5b7f4bb0bc077e62c39258067e17c0094
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>

vcl/source/opengl/OpenGLContext.cxx

@@ -1397,7 +1397,7 @@ bool OpenGLContext::isCurrent()
     return false;
 #elif defined( UNX )
     GLXDrawable nDrawable = mbPixmap ? m_aGLWin.glPix : m_aGLWin.win;
-    return (glXGetCurrentContext() == m_aGLWin.ctx &&
+    return (m_aGLWin.ctx && glXGetCurrentContext() == m_aGLWin.ctx &&
             glXGetCurrentDrawable() == nDrawable);
 #endif
 }