glx: Unconditionally clear lastGLContext on loseCurrent().

This hook calls unbindContext in the DRI driver interface, which
unsets the dispatch table, regardless of whether the context argument
was the current one or not.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

glx/glxext.c

@@ -135,8 +135,7 @@ DrawableGone(__GLXdrawable * glxPriv, XID xid)
 		(c->drawPriv == glxPriv || c->readPriv == glxPriv)) {
             /* just force a re-bind the next time through */
             (*c->loseCurrent) (c);
-            if (c == lastGLContext)
-                lastGLContext = NULL;
+            lastGLContext = NULL;
         }
         if (c->drawPriv == glxPriv)
             c->drawPriv = NULL;
@@ -290,6 +289,7 @@ glxClientCallback(CallbackListPtr *list, void *closure, void *data)
             next = c->next;
             if (c->currentClient == pClient) {
                 c->loseCurrent(c);
+                lastGLContext = NULL;
                 c->currentClient = NULL;
                 __glXFreeContext(c);
             }