Bug #30499288 - GCC 9.2.1 REPORTS A NEW WARNING FOR OS_FILE_GET_PARENT_DIR

- Fixed a warning visible in optimized build related to calling
memcpy with length parameters larger than ptrdiff_t max.

rb#23333 approved by Annamalai Gurusami <annamalai.gurusami@oracle.com>

storage/innobase/os/os0file.cc

 /***********************************************************************
 
-Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 1995, 2019, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2009, Percona Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted
 by Percona Inc.. Those modifications are
@@ -1474,6 +1474,12 @@ os_file_get_parent_dir(
 		return(NULL);
 	}
 
+	if (last_slash - path < 0) {
+		/* Sanity check, it prevents gcc from trying to handle this case which
+		 * results in warnings for some optimized builds */
+		return (NULL);
+	}
+
 	/* Non-trivial directory component */
 
 	return(mem_strdupl(path, last_slash - path));