Bug #28178776 COMPARISON OF UNINITAILIZED MEMORY IN LOG_IN_USE

PROBLEM
-------
Memory sanitizer reports uninitialized comparisons
in log_in_use(), because strings are compared with
memcmp() instead of strncmp.

FIX
---
Use strncmp() to compare strings

sql/log.cc

-/* Copyright (c) 2000, 2016, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2016, MariaDB
+/* Copyright (c) 2000, 2018, Oracle and/or its affiliates.
+   Copyright (c) 2009, 2019, MariaDB Corporation
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -3413,7 +3413,7 @@ int MYSQL_BIN_LOG::find_log_pos(LOG_INFO *linfo, const char *log_name,
     // if the log entry matches, null string matching anything
     if (!log_name ||
 	(log_name_len == fname_len-1 && full_fname[log_name_len] == '\n' &&
-	 !memcmp(full_fname, full_log_name, log_name_len)))
+	 !strncmp(full_fname, full_log_name, log_name_len)))
     {
       DBUG_PRINT("info", ("Found log file entry"));
       full_fname[fname_len-1]= 0;			// remove last \n

sql/sql_repl.cc

-/* Copyright (c) 2000, 2017, Oracle and/or its affiliates.
-   Copyright (c) 2008, 2017, MariaDB Corporation
+/* Copyright (c) 2000, 2018, Oracle and/or its affiliates.
+   Copyright (c) 2008, 2019, MariaDB Corporation
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -365,7 +365,7 @@ bool log_in_use(const char* log_name)
     if ((linfo = tmp->current_linfo))
     {
       mysql_mutex_lock(&linfo->lock);
-      result = !memcmp(log_name, linfo->log_file_name, log_name_len);
+      result = !strncmp(log_name, linfo->log_file_name, log_name_len);
       mysql_mutex_unlock(&linfo->lock);
       if (result)
 	break;