XB crashes with insufficient param.

1. This came up while experimenting this with PXC

2. XB has 3 step process. backup -> prepare -> move-back

3. Error occurs with move-back

4. PXC accidentally invoke XB with following params

innobackupex --defaults-file= --defaults-group=mysqld.2 --no-version-check --move-back --force-non-empty-directories ${DATA} &>${DATA}/innobackup.move.log

Note: defaults-file is empty and there is no datadir causing XB to assume that source = destination

and this eventually caused the error/crash as follows:
innobackupex version 2.4.0-rc1 based on MySQL server 5.7.10 Linux (x86_64) (revision id: 0d11984)
Error: Move file ib_logfile0 to ./ib_logfile0 failed: Destination file exists
*** Error in `innobackupex': free(): invalid pointer: 0x00007ffcf7c6042c ***
03:46:27 UTC - xtrabackup got signal 6 ;
This could be because you hit a bug or data is corrupted.
This error can also be caused by malfunctioning hardware.
Attempting to collect some information that could help diagnose the problem.
As this is a crash and something is definitely wrong, the information
collection process might fail.

Should be reproducible with standalone XB too.

Observed with XB-2.4.1 (may exist with other versions too)