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.

libctf/ctf-create.c

@@ -82,7 +82,7 @@ ctf_create (int *errp)
   fp->ctf_dvhash = dvhash;
   fp->ctf_dtnextid = 1;
   fp->ctf_dtoldid = 0;
-  fp->ctf_snapshots = 0;
+  fp->ctf_snapshots = 1;
   fp->ctf_snapshot_lu = 0;
 
   return fp;