windows opengl: Avoid an unnecessary wglMakeCurrent() call.

Change-Id: I1c1287a29067c053565c49c0ce96918a2d27d87f
Reviewed-on: https://gerrit.libreoffice.org/18260Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

vcl/source/opengl/OpenGLContext.cxx

@@ -1278,7 +1278,6 @@ void OpenGLContext::reset()
     // destroy all programs
     if( !maPrograms.empty() )
     {
-
         makeCurrent();
         maPrograms.clear();
     }
@@ -1296,7 +1295,8 @@ void OpenGLContext::reset()
         if (itr != g_vShareList.end())
             g_vShareList.erase(itr);
 
-        wglMakeCurrent(NULL, NULL);
+        if (wglGetCurrentContext() != NULL)
+            wglMakeCurrent(NULL, NULL);
         wglDeleteContext( m_aGLWin.hRC );
         ReleaseDC( m_aGLWin.hWnd, m_aGLWin.hDC );
     }