Handle NULL string argument to foreach

src/vmod_taskvar.c

@@ -115,7 +115,7 @@ vmod_string_foreach(VRT_CTX, struct VPFX(taskvar_string) *va,
 	struct vmod_taskvar_string *v =
 	    state_l(ctx, va, sizeof *v);
 
-	if (v == NULL)
+	if (v == NULL || s == NULL)
 		return;
 
 	CHECK_OBJ(v, VMOD_TASKVAR_STRING_MAGIC);

src/vtc/vmod_taskvar_foreach.vtc

@@ -18,6 +18,7 @@ varnish v1 -vcl {
     }
 
     sub vcl_recv {
+	str.foreach(req.http.null, iter);
 	str.foreach(req.http.foo, iter);
 	return (synth(200));
     }
@@ -48,4 +49,4 @@ client c1 {
     expect resp.status == 404
 } -run
 
-logexpect l1 -wait
\ No newline at end of file
+logexpect l1 -wait