store() method emits and error to the log and returns if the string

to be stored is NULL

src/tests/b03.vtc

+varnishtest "vtstor-vmod: test error handling"
+
+server s1 {} -start
+
+varnish v1 -vcl+backend {
+	import vtstor from "${vmod_topbuild}/src/.libs/libvmod_vtstor.so";
+        import std;
+
+        sub vcl_init {
+        	new zero = vtstor.vtstor(0, 0, 0, 0);
+        }
+
+        sub vcl_recv {
+	    zero.store(req.http.Foo, req.http.Bar);
+            return(synth(200, "OK"));
+        }
+} -start
+
+client c1 {
+	txreq -hdr "Bar: baz"
+	rxresp
+	expect resp.status == 200
+} -run
+
+logexpect l1 -v v1 -d 1 -g vxid -q "VCL_Error" {
+	expect 0 * Begin req
+	expect * = VCL_Error ^vmod vtstor: store() method called with NULL string$
+	expect * = End
+} -run

src/vmod_vtstor.c

@@ -114,6 +114,12 @@ vmod_vtstor_store(VRT_CTX, struct vmod_vtstor_vtstor *vmi,
 	AN(hdr);
 	AN(hdr->what);
 
+	if (s == NULL) {
+		VSLb(ctx->vsl, SLT_VCL_Error,
+		     "vmod vtstor: store() method called with NULL string");
+		return;
+	}
+
 	hp = selecthttp(ctx, hdr->where);
 	CHECK_OBJ_NOTNULL(hp, HTTP_MAGIC);