LP #1590351: xtrabackup --backup crashes with empty keyring file

**Reported in Launchpad by Sergei Glushchenko last update 11-07-2016 10:00:11

Simple test case

touch keyring_file

./xtrabackup --backup --datadir=./dat1 --target-dir=./backup --user=root --keyring-file-data=keyring_file