Handle VRT_priv_task errors gracefully

The reason is the same as for
https://github.com/varnishcache/varnish-cache/issues/2708 :

VRT_priv_task may now return NULL for out-of-workspace errors, see
https://github.com/varnishcache/varnish-cache/commit/de2b431086a13b243dc8b3e71cd8697db1df2c7f

src/vmod_re.c

@@ -286,6 +286,12 @@ vmod_regex_match(VRT_CTX, struct vmod_re_regex *re, VCL_STRING subject,
 	AN(re->vre);
 
 	task = VRT_priv_task(ctx, re);
+
+	if (task == NULL) {
+		errmsg(ctx, "vmod re: no priv - out of workspace?");
+		return (0);
+	}
+
 	AN(task);
 	task->len = 0;
 
@@ -302,6 +308,10 @@ vmod_regex_backref(VRT_CTX, struct vmod_re_regex *re, VCL_INT refnum,
 	CHECK_OBJ_NOTNULL(re, VMOD_RE_REGEX_MAGIC);
 
 	task = VRT_priv_task(ctx, re);
+	if (task == NULL) {
+		errmsg(ctx, "vmod re: no priv - out of workspace?");
+		return (0);
+	}
 	return backref(ctx, refnum, fallback, task);
 }