nodedevmdevctltest: Fix two memleaks

nodedevmdevctltest: Fix two memleaks

There are two memleaks inside of nodedevmdevctltest:

1) In the testCommandDryRunCallback() - when appending lines to
   stdinbuf the pointer is overwritten without freeing the old
   memory it pointed to.

2) In testMdevctlModify() the livecmd variable is reused and
   since its marked as g_autoptr() the first use leaks.

Fixes: 582f27ff15f14fa0e913278c0a2ca9840c3f12f5
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>