Crash after a call to pg_backup_start()

While playing with a proposed patch, I noticed that a session crashes
after a failed call to pg_backup_start().

postgres=# select pg_backup_start(repeat('x', 1026));
ERROR: backup label too long (max 1024 bytes)
postgres=# \q
> TRAP: failed Assert("during_backup_start ^ (sessionBackupState == SESSION_BACKUP_RUNNING)"), File: "xlog.c", Line: 8846, PID: 165835

Surprisingly this happens by a series of succussful calls to
pg_backup_start and stop.

postgres=# select pg_backup_start('x');
postgres=# select pg_backup_top();
postgres=# \q
> TRAP: failed Assert("durin..

>> do_pg_abort_backup(int code, Datum arg)
> /* Only one of these conditions can be true */
> Assert(during_backup_start ^
> (sessionBackupState == SESSION_BACKUP_RUNNING));

It seems to me that the comment is true and the condition is a thinko.
This is introduced by df3737a651 so it is master only.

Please find the attached fix.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center