xaa: avoid possible freed pointer reuse in epilogue

If the pGCPriv->flags == 2, then we try to assign the freed pGCPriv->XAAOps
avoid this by clearing the flags in to be destroyed pGCPriv.

Reported by coverity.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>

hw/xfree86/xaa/xaaGC.c

@@ -239,6 +239,7 @@ XAADestroyGC(GCPtr pGC)
 	free(pGCPriv->XAAOps);
 
     free(pGCPriv->DashPattern);
+    pGCPriv->flags = 0;
 
     (*pGC->funcs->DestroyGC)(pGC);
     XAA_GC_FUNC_EPILOGUE (pGC);