Fix regex matches in some esoteric cases

E.g. set req.http.foo = req.http.foo + req.http.bar ~ "foobar"

bin/varnishtest/tests/v00020.vtc

@@ -120,10 +120,8 @@ varnish v1 -vcl {
 		set req.http.foo = 3.0 * 2;
 		set req.http.foo = 3.0 / 2;
 
-		set req.http.foo = req.http.foo ~ "bar";
-		set req.http.foo = req.http.foo !~ "bar";
-		set req.http.foo = "foo" ~ "bar";
-		set req.http.foo = "foo" !~ "bar";
+		set req.http.foo = req.http.foo + "bar" ~ "bar";
+		set req.http.foo = req.http.foo + "bar" !~ "bar";
 
 		set req.ttl = 1s;
 	}

lib/libvcc/vcc_expr.c

@@ -1147,6 +1147,8 @@ vcc_expr_cmp(struct vcc *tl, struct expr **e, vcc_type_t fmt)
 	}
 	if (((*e)->fmt == STRING || (*e)->fmt == STRING_LIST) &&
 	    (tl->t->tok == '~' || tl->t->tok == T_NOMATCH)) {
+		if ((*e)->fmt == STRING_LIST)
+			vcc_expr_tostring(tl, e, STRING);
 		not = tl->t->tok == '~' ? "" : "!";
 		vcc_NextToken(tl);
 		ExpectErr(tl, CSTR);