lockdown: return LOCKDOWN_E_INVALID_HOST_ID when missing pair record

When the check of /var/db/lockdown was removed, lockdownd_do_pair started to
always return LOCKDOWN_E_INVALID_CONF instead of usually (but not always...)
returning LOCKDOWN_E_INVALID_HOST_ID for devices not currently paired. This
change not only breaks some third-party code, but also breaks the other code
in this library calling this function (lockdownd_client_new_with_handshake).
diff --git a/src/lockdown.c b/src/lockdown.c
index 904267e..cae950b 100644
--- a/src/lockdown.c
+++ b/src/lockdown.c
@@ -895,7 +895,7 @@
 			/* use existing pair record */
 			userpref_read_pair_record(client->udid, &pair_record_plist);
 			if (!pair_record_plist) {
-				return LOCKDOWN_E_INVALID_CONF;
+				return LOCKDOWN_E_INVALID_HOST_ID;
 			}
 		}
 	}