sequencer.c: check for lock failure and bail early in fast_forward_to

Change fast_forward_to() to check if locking the ref failed, print a
nice error message and bail out early.

The old code did not check if ref_lock was NULL and relied on the
fact that the write_ref_sha1() would safely detect this condition
and set the return variable ret to indicate an error.

While that is safe, it makes the code harder to read for two reasons:

 * Inconsistency.  Almost all other places we do check the lock for
   NULL explicitly, so the naive reader is confused "why don't we
   check here?"

 * And relying on write_ref_sha1() to detect and return an error for
   when a previous lock_any_ref_for_update() failed feels obfuscated.

This change should not change any functionality or logic aside from
adding an extra error message when this condition is triggered
(write_ref_sha1() returns an error silently for this condition).

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -281,8 +281,12 @@ static int fast_forward_to(const unsigned char *to, const unsigned char *from,
 		exit(1); /* the callee should have complained already */
 	ref_lock = lock_any_ref_for_update("HEAD", unborn ? null_sha1 : from,
 					   0, NULL);
+	if (!ref_lock)
+		return error(_("Failed to lock HEAD during fast_forward_to"));
+
 	strbuf_addf(&sb, "%s: fast-forward", action_name(opts));
 	ret = write_ref_sha1(ref_lock, to, sb.buf);
+
 	strbuf_release(&sb);
 	return ret;
 }