Fix the error message when the path in the VDP constructor is empty.

src/tests/vdp.vtc

@@ -138,6 +138,15 @@ varnish v1 -vcl+backend {
 
 client c1 -run
 
+varnish v1 -errvcl {vdp pipe failure: new emptypath: path is empty} {
+	import ${vmod_pipe};
+	backend b None;
+
+	sub vcl_init {
+		new emptypath = pipe.vdp(path="");
+	}
+}
+
 varnish v1 -vcl { backend b None; }
 
 # Tests the discard event, with removal of VDPs.

src/vdfp_pipe.c

@@ -540,7 +540,7 @@ vmod_vdp__init(VRT_CTX, struct VPFX(pipe_vdp) **vdpp, const char *obj_name,
 	AN(obj_name);
 
 	if (path == NULL || *path == '\0') {
-		VDPFAIL(ctx, "new %s: path is empty", path);
+		VDPFAIL(ctx, "new %s: path is empty", obj_name);
 		return;
 	}
 	errno = 0;