Fix 'Caught unhandled unknown exception; terminating' bug in SQL steps

[tkren]

Running pgagent 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.

This pull request 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...
[...]

[repo-lockdown]

Thanks for your Pull Request! 😄 This repo on GitHub is just a mirror of our real git repositories though, and can't really handle PRs. 😦 Hopefully you can redo the PR, and direct it to the git.postgresql.org repos? We have a developer guide, if that helps: https://wiki.postgresql.org/wiki/So,_you_want_to_be_a_developer%3F. If this was a PR for pgAdmin, please visit https://www.pgadmin.org/docs/pgadmin4/dev/submitting_patches.html.