libctf: fix spurious error when rolling back to the first snapshot

libctf: fix spurious error when rolling back to the first snapshot

The first ctf_snapshot called after CTF file creation yields a snapshot
handle that always yields a spurious ECTF_OVERROLLBACK error ("Attempt
to roll back past a ctf_update") on ctf_rollback(), even if ctf_update
has never been called.

The fix is to start with a ctf_snapshot value higher than the zero value
that ctf_snapshot_lu ("last update CTF snapshot value") is initialized
to.

libctf/
	* ctf-create.c (ctf_create): Fix off-by-one error.