vmod_vtc: Add support for thread workspace operations on the backend side

This is probably useless at the moment because of how the vcl method code checks
thread workspace allocations, but at least we avoid a panic at the wrong place.

vmod/vmod_vtc.c

@@ -142,8 +142,10 @@ vtc_ws_find(VRT_CTX, VCL_ENUM which)
 		return (ctx->bo->ws);
 	if (which == VENUM(session))
 		return (ctx->req->sp->ws);
-	if (which == VENUM(thread))
+	if (which == VENUM(thread) && ctx->req != NULL)
 		return (ctx->req->wrk->aws);
+	if (which == VENUM(thread) && ctx->bo != NULL)
+		return (ctx->bo->wrk->aws);
 	WRONG("vtc_ws_find Illegal enum");
 }