PATCH: pgagent fix for 'Caught unhandled unknown exception; terminating' bug in SQL steps

Hi!

Running pgagent 3.4.0 on Debian jessie with an SQL jobstep crashes the
worker thread with the error message

Caught unhandled unknown exception; terminating

Both job and jobstep status then remain as 'r' (running) until the
pgagent process quits. After restarting pgagent, which cleans up the
job status and sets it to 'd' (aborted), the job containing the SQL
jobstep is free to run again, but the next run will end up in the same
deadlock situation.

I have tracked down the problem to DBconn::GetLastError(), which gets
called after each SQL jobstep execution in Job::Execute(): there,
DBconn::GetLastError() throws an exception whenever the last error
message is empty.

The attached patch fixes this problem by adding missing bounds checks
to DBconn::GetLastError().

Here is a log of pgagent with an SQL jobstep:

$ pgagent -l 2 -f hostaddr=127.0.0.1 port=5432 dbname=pgdb user=pgdb
DEBUG: Creating primary connection
DEBUG: Connection Information:
DEBUG:      user         : pgdb
DEBUG:      port         : 5432
DEBUG:      host         : 127.0.0.1
DEBUG:      dbname       : pgdb
DEBUG:      password     :
DEBUG:      conn timeout : 0
DEBUG: Connection Information:
DEBUG:      user         : pgdb
DEBUG:      port         : 5432
DEBUG:      host         : 127.0.0.1
DEBUG:      dbname       : pgdb
DEBUG:      password     :
DEBUG:      conn timeout : 0
DEBUG: Creating DB connection: user=pgdb port=5432 hostaddr=127.0.0.1 dbname=pgdb
DEBUG: Database sanity check
DEBUG: Clearing zombies
DEBUG: Checking for jobs to run
DEBUG: Sleeping...
DEBUG: Clearing inactive connections
DEBUG: Connection stats: total - 1, free - 0, deleted - 0
DEBUG: Checking for jobs to run
DEBUG: Sleeping...
[...]
DEBUG: Clearing inactive connections
DEBUG: Connection stats: total - 1, free - 0, deleted - 0
DEBUG: Checking for jobs to run
DEBUG: Sleeping...
DEBUG: Connection stats: total - 1, free - 0, deleted - 0
DEBUG: Checking for jobs to run
DEBUG: Creating job thread for job 5
DEBUG: Creating DB connection: user=pgdb port=5432 hostaddr=127.0.0.1 dbname=pgdb
DEBUG: Allocating new connection to database pgdb
DEBUG: Starting job: 5
DEBUG: Sleeping...
DEBUG: Creating DB connection: user=pgdb port=5432 hostaddr=127.0.0.1 dbname=pgdb dbname=pgdb
DEBUG: Allocating new connection to database pgdb
DEBUG: Executing SQL step 40 (part of job 5)
*** Caught unhandled unknown exception; terminating
DEBUG: Destroying job thread for job 5
DEBUG: Checking for jobs to run
DEBUG: Sleeping...
DEBUG: Clearing inactive connections
DEBUG: Connection stats: total - 3, free - 0, deleted - 0
DEBUG: Checking for jobs to run
DEBUG: Sleeping...
DEBUG: Clearing inactive connections
DEBUG: Connection stats: total - 3, free - 0, deleted - 0
DEBUG: Checking for jobs to run
DEBUG: Sleeping...
[...]