pgAgent crashes on failed connection

I created 100 identical pgagent jobs, with one step that simply does
"SELECT pg_sleep(10)". I then forced them all to run immediately, with
"UPDATE pgagent.pga_job SET jobnextrun=now();". pgagent crashed.

What happened is that the when all those jobs are launched at the same
time, the server ran into the max_connections limit, and pgagent didn't
handle that too well. JobThread::JobThread constructor does not check
for NULL result from DBConn::Get(), and passes a NULL connection to
Job::Job, which tries to reference it, leading to a segfault.

I propose the attached patch.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com