realtime: Fix ast_update2_realtime() on raspberry pi.

The old code depended on undefined va_arg behaviour: calling a function
twice with the same va_list parameter and expecting it to continue where
it left off. The changed code behaves like the manpage says it should.

Also added a bunch of early returns to trap errors (e.g. OOM) instead of
crashing.

The problem was found by Julian Lyndon-Smith. The deviant behaviour on
the raspberry PI also uncovered another bug (fixed in r407875) in the
res_config_pgsql.so driver.

Reported by: jmls
Tested by: jmls
Review: https://reviewboard.asterisk.org/r/3201/
........

Merged revisions 407968 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@407970 f38db490-d61c-443f-a65b-d21fe96a405b