glx: Use 0 rather than garbage for unknown INTEL_swap_event types

Otherwise the garbage you return could well be numerically identical to
one of the swap type tokens, and apps which rely on us to tell the truth
would be in trouble.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit f8540b9dcc)

glx/glxdri2.c

@@ -184,6 +184,7 @@ __glXdriSwapEvent(ClientPtr client, void *data, int type, CARD64 ust,
 	break;
     default:
 	/* unknown swap completion type */
+	wire.event_type = 0;
 	break;
     }
     wire.drawable = drawable->drawId;