qemu: Resolve Coverity FORWARD_NULL

qemu: Resolve Coverity FORWARD_NULL

If we end up at the cleanup lable before we've VIR_EXPAND_N the list,
then calling virQEMUCapsFreeStringList() with a NULL proplist could
theoretically deref proplist if nproplist was set. Coverity doesn't
seem to acknowledge the relationship between proplist and nproplist
assuming in virQEMUCapsFreeStringList that nproplist could be at
least 1 and thus have a null deref.  It only seems to follow the
NULL proplist.

Signed-off-by: John Ferlan <jferlan@redhat.com>