assert for VGZ_NewGzip() failures

VGZ_NewGzip will either assert or succeed.

bin/varnishd/cache/cache_esi_fetch.c

@@ -170,6 +170,7 @@ vfp_esi_gzip_init(struct vfp_ctx *vc, struct vfp_entry *vfe)
 		return (VFP_ERROR);
 	}
 	vef->vgz = VGZ_NewGzip(vc->wrk->vsl, "G F E");
+	AN(vef->vgz);
 
 	vef->ibuf_sz = cache_param->gzip_buffer;
 	vef->ibuf = calloc(1L, vef->ibuf_sz);

bin/varnishd/cache/cache_gzip.c

@@ -479,8 +479,7 @@ vfp_gzip_init(struct vfp_ctx *vc, struct vfp_entry *vfe)
 			vc->obj_flags |= OF_GZIPED;
 		}
 	}
-	if (vg == NULL)
-		return (VFP_ERROR);
+	AN(vg);
 	vfe->priv1 = vg;
 	if (vgz_getmbuf(vg))
 		return (VFP_ERROR);