multi: during ratelimit multi_getsock should return no sockets

... as there is nothing to wait for then, it just waits. Otherwise, this
causes much more CPU work and updates than necessary during ratelimit
periods.

Ref: https://curl.se/mail/lib-2023-11/0056.html
Closes #12430

lib/multi.c

@@ -1064,6 +1064,10 @@ static void multi_getsock(struct Curl_easy *data,
   case MSTATE_PERFORMING:
     Curl_pollset_add_socks(data, ps, Curl_single_getsock);
     break;
+
+  case MSTATE_RATELIMITING:
+    /* nothing to wait for */
+    return;
   }
 
   /* Let connection filters add/remove as needed */