Fix GL context destruction with AIGLX.

The logic for freeing GL contexts introduced by "Fix AIGLX VT switching." is
inverted. As it is now, GL context destruction is deferred for glxDRIEnterVT().

GL/glx/glxext.c

@@ -237,7 +237,7 @@ GLboolean __glXFreeContext(__GLXcontext *cx)
      * __glXDispatch() or as a callback from the resource manager.  In
      * the latter case we need to lift the DRI lock manually. */
 
-    if (glxBlockClients) {
+    if (!glxBlockClients) {
 	__glXleaveServer();
 	cx->destroy(cx);
 	__glXenterServer();