fix forgotten WS_Release() call for failing WS_Reserve()

See also https://github.com/varnishcache/varnish-cache/issues/2967
for a general discussion regarding an inconsistency in the WS_Reserve()
interface

src/vmod_selector.c

@@ -455,6 +455,7 @@ vmod_set_hasprefix(VRT_CTX, struct vmod_selector_set *set, VCL_STRING subject)
 	if ((match->limit = WS_ReserveLumps(ctx->ws, sizeof(unsigned))) == 0) {
 		VERRNOMEM(ctx, "Reserving index array in "
 			  "%s.hasprefix(\"%.40s\")", set->vcl_name, subject);
+		WS_Release(ctx->ws, 0);
 		return (0);
 	}
 	match->indices = (unsigned *)WS_Front(ctx->ws);