nodelist: use strcmpstart() instead of strncmp()

in rend_parse_v2_service_descriptor.
Fix on c58675ca72

changes/ticket27630

@@ -0,0 +1,3 @@
+  o Code simplification and refactoring:
+    - Use the simpler strcmpstart() helper in rend_parse_v2_service_descriptor
+      instead of strncmp(). Closes ticket 27630.

src/feature/nodelist/routerparse.c

@@ -5167,8 +5167,7 @@ rend_parse_v2_service_descriptor(rend_service_descriptor_t **parsed_out,
 
   tor_assert(desc);
   /* Check if desc starts correctly. */
-  if (strncmp(desc, "rendezvous-service-descriptor ",
-              strlen("rendezvous-service-descriptor "))) {
+  if (strcmpstart(desc, "rendezvous-service-descriptor ")) {
     log_info(LD_REND, "Descriptor does not start correctly.");
     goto err;
   }