MDEV-7429 main.mysqldump fails sporadically in buildbot

prevent concurrent cleanups in multi-threaded mysqlimport
(they can happen if many threads get an error at the same time),
safe_exit() is not thread-safe.

client/mysqlimport.c

@@ -36,7 +36,7 @@
 
 
 /* Global Thread counter */
-uint counter;
+uint counter= 0;
 pthread_mutex_t counter_mutex;
 pthread_cond_t count_threshhold;
 
@@ -489,6 +489,11 @@ static void safe_exit(int error, MYSQL *mysql)
 {
   if (error && ignore_errors)
     return;
+
+  /* in multi-threaded mode protect from concurrent safe_exit's */
+  if (counter)
+    pthread_mutex_lock(&counter_mutex);
+
   if (mysql)
     mysql_close(mysql);