vsh: cmdComplete: Don't leak buffer for completion

The buffer which we assign to the 'rl_line_buffer' variable of readline
would be overwritten and thus leaked on multiple invocations of
cmdComplete in one session.

Free/clear it after it's used.

Hitting this leak was until recenly possible only in non-interactive
batch mode and recently also in interactive mode as 'complete' can be
used multiple times now interactively.

Fixes: a0e1ada6
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>