Removed duplicate thread cache check

It may be beneficial for slow-path, where new thread is to be created.
However it is harmful for fast-path, where thread cache can fulfil
request: it caused 2 extra loads of write-contended variables.

Also shrink variables scope.

Part of MDEV-19515 - Improve connect speed

sql/mysqld.cc

@@ -6188,15 +6188,9 @@ void handle_connection_in_main_thread(CONNECT *connect)
 
 void create_thread_to_handle_connection(CONNECT *connect)
 {
-  char error_message_buff[MYSQL_ERRMSG_SIZE];
-  int error;
   DBUG_ENTER("create_thread_to_handle_connection");
 
-  /* Check if we can get thread from the cache */
-  if (cached_thread_count > wake_thread)
-  {
   mysql_mutex_lock(&LOCK_thread_cache);
-    /* Recheck condition when we have the lock */
   if (cached_thread_count > wake_thread)
   {
     /* Get thread from cache */
@@ -6208,17 +6202,17 @@ void create_thread_to_handle_connection(CONNECT *connect)
     DBUG_VOID_RETURN;
   }
   mysql_mutex_unlock(&LOCK_thread_cache);
-  }
 
   /* Create new thread to handle connection */
   inc_thread_created();
   DBUG_PRINT("info",(("creating thread %lu"), (ulong) connect->thread_id));
   connect->prior_thr_create_utime= microsecond_interval_timer();
 
-  if ((error= mysql_thread_create(key_thread_one_connection,
+  if (auto error= mysql_thread_create(key_thread_one_connection,
                                       &connect->real_id, &connection_attrib,
-                                  handle_one_connection, (void*) connect)))
+                                      handle_one_connection, (void*) connect))
   {
+    char error_message_buff[MYSQL_ERRMSG_SIZE];
     /* purecov: begin inspected */
     DBUG_PRINT("error", ("Can't create thread to handle request (error %d)",
                 error));