ecpg: Fix out-of-bound read in DecodeDateTime()

ecpg: Fix out-of-bound read in DecodeDateTime()

It was possible for the code to read out-of-bound data from the
"day_tab" table with some crafted input data.  Let's treat these as
invalid input as the month number is incorrect.

A test is added to test this case with a check on the errno returned by
the decoding routine.  A test close to the new one added in this commit
was testing for a failure, but did not look at the errno generated, so
let's use this commit to also change it, adding a check on the errno
returned by DecodeDateTime().

Like the other test scripts, dt_test should likely be expanded to
include more checks based on the errnos generated in these code paths.
This is left as future work.

This issue exists since 2e6f97560a83, so backpatch all the way down.

Reported-by: Pavel Nekrasov
Author: Bruce Momjian, Pavel Nekrasov
Discussion: https://postgr.es/m/18614-6bbe00117352309e@postgresql.org
Backpatch-through: 12