pgsql: Initialize FOUND to false in PL/pgSQL event trigger functions.

Initialize FOUND to false in PL/pgSQL event trigger functions.

The documentation says that FOUND starts out false within each
PL/pgSQL function call. plpgsql_exec_function() and
plpgsql_exec_trigger() do set it so, but plpgsql_exec_event_trigger()
did not, leaving it NULL until the first statement that sets it.

Fix this by initializing FOUND to false, as the other two entry points
do. While at it, also set estate.err_text to "during function entry"
during this action and the subsequent instrumentation plugin call,
again to match the other two routines.

Back-patch to v19, but no further. It seems unlikely that any user
code is depending on the current behavior, but just in case, let's
not touch the stable branches.

Author: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAM2+6=U_=hZmgs1v1JJ-XJk2bpbfqQPP9MNRDbp5RoEsOW8xsQ@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/56d9ecf6bd882a16afbeae7b8f740f9febcf420c

Modified Files
--------------
src/pl/plpgsql/src/pl_exec.c | 7 +++++++
1 file changed, 7 insertions(+)