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/set.c

@@ -348,7 +348,10 @@ vmod_set_match(VRT_CTX, struct vmod_re2_set *set, VCL_STRING subject)
 	}
 
 	priv = VRT_priv_task(ctx, set);
-	AN(priv);
+	if (priv == NULL) {
+		ERR(ctx, "No priv_task - workspace overflow?");
+		return 0;
+	}
 	if (priv->priv == NULL) {
 		if ((priv->priv = WS_Alloc(ctx->ws, sizeof(*task))) == NULL) {
 			VERRNOMEM(ctx, ERR_PREFIX "allocating match data",
@@ -408,8 +411,7 @@ get_task_data(VRT_CTX, struct vmod_re2_set *set)
 	struct task_set_match *task;
 
 	priv = VRT_priv_task(ctx, set);
-	AN(priv);
-	if (priv->priv == NULL)
+	if (priv == NULL || priv->priv == NULL)
 		return NULL;
 	WS_Assert_Allocated(ctx->ws, priv->priv, sizeof(*task));
 	CAST_OBJ(task, priv->priv, TASK_SET_MATCH_MAGIC);

src/vmod_re2.c

@@ -301,7 +301,10 @@ vmod_regex_match(const struct vrt_ctx *ctx, struct vmod_re2_regex *re,
 	CHECK_OBJ_NOTNULL(re, VMOD_RE2_REGEX_MAGIC);
 
 	task = VRT_priv_task(ctx, re);
-	AN(task);
+	if (task == NULL) {
+		ERR(ctx, "No priv_task - workspace overflow?");
+		return 0;
+	}
 	if (task->priv == NULL) {
 		if ((task->priv = WS_Alloc(ctx->ws, sizeof(*task_match)))
 		    == NULL) {
@@ -355,7 +358,10 @@ vmod_regex_backref(VRT_CTX, struct vmod_re2_regex *re, VCL_INT refnum,
 	}
 
 	task = VRT_priv_task(ctx, re);
-	AN(task);
+	if (task == NULL) {
+		ERR(ctx, "No priv_task - workspace overflow?");
+		return 0;
+	}
 	if (task->priv == NULL) {
 		VERR(ctx, ERR_PREFIX "backref called without prior match",
 		     re->vcl_name, refnum, fallback);
@@ -396,7 +402,10 @@ vmod_regex_namedref(VRT_CTX, struct vmod_re2_regex *re, VCL_STRING name,
 	}
 
 	task = VRT_priv_task(ctx, re);
-	AN(task);
+	if (task == NULL) {
+		ERR(ctx, "No priv_task - workspace overflow?");
+		return 0;
+	}
 	if (task->priv == NULL) {
 		VERR(ctx, ERR_PREFIX "namedref called without prior match",
 		     re->vcl_name, name, fallback);