Fix vcl.show for labels

When executing vcl.show for labels, show the VCL the label points to
instead of asserting.

bin/varnishd/cache/cache_vcl.c

@@ -876,6 +876,11 @@ vcl_cli_show(struct cli *cli, const char * const *av, void *priv)
 		return;
 	}
 	CHECK_OBJ_NOTNULL(vcl, VCL_MAGIC);
+	if (vcl->label) {
+		vcl = vcl->label;
+		CHECK_OBJ_NOTNULL(vcl, VCL_MAGIC);
+		AZ(vcl->label);
+	}
 	CHECK_OBJ_NOTNULL(vcl->conf, VCL_CONF_MAGIC);
 	if (verbose) {
 		for (i = 0; i < vcl->conf->nsrc; i++)

bin/varnishtest/tests/s00005.vtc

@@ -40,6 +40,8 @@ varnish v1 -cliok "vcl.label foo vcl2"
 varnish v1 -cliok "vcl.label bar vcl2"
 varnish v1 -cliok "vcl.list"
 varnish v1 -clijson "vcl.list -j"
+varnish v1 -cliok "vcl.show foo"
+varnish v1 -cliok "vcl.show -v bar"
 varnish v1 -clierr 300 "vcl.discard vcl2"
 varnish v1 -cliok "vcl.discard bar"
 varnish v1 -cliok "vcl.label foo vcl1"