glx: Be a bit more paranoid in glx client cleanup

This would probably crash (via double-free) if you had multiple GPUs and
an indirect context.

Signed-off-by: Adam Jackson <ajax@redhat.com>

glx/glxext.c

@@ -293,6 +293,7 @@ glxClientCallback(CallbackListPtr *list, void *closure, void *data)
         free(cl->returnBuf);
         free(cl->largeCmdBuf);
         free(cl->GLClientextensions);
+        memset(cl, 0, sizeof(*cl));
         break;
 
     default: