agent: Fix a race condition which results accessing finished scd.

* agent/call-daemon.c (daemon_start): Decision of connection/reuse of
CTX and assignment to ->ctx should be done with the lock.

When scdaemon is exiting and agent tries to spawn/connect/reconnect,
there is a race condition between detecting finish of scd and
spawn/connect/reconnect.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>