if we have enough usable guards, just pick one

we don't need to check whether we don't have enough guards right after
concluding that we do have enough.

slight efficiency fix suggested by an anonymous fellow on irc.

src/or/circuitbuild.c

@@ -4186,7 +4186,7 @@ choose_random_entry(cpath_build_state_t *state)
         goto choose_and_finish;
       }
       if (smartlist_len(live_entry_guards) >= options->NumEntryGuards)
-        break; /* we have enough */
+        goto choose_and_finish; /* we have enough */
   } SMARTLIST_FOREACH_END(entry);
 
   if (entry_list_is_constrained(options)) {