MDEV-14337 mysqld_safe may suppress error messages with --log-output=file

don't close stdout/stderr, redirect them to /dev/null instead.
otherwise redirections like >&2 fail with "invalid file descriptor"

scripts/mysqld_safe.sh

@@ -868,8 +868,8 @@ if expr "${-}" : '.*x' > /dev/null
 then
   :
 else
-  exec 1>&-
-  exec 2>&-
+  exec 1>/dev/null
+  exec 2>/dev/null
 fi
 
 while true