BUG #18799: /src/interfaces/ecpg/pgtypeslib/timestamp.c possible NULL deref

The following bug has been logged on the website:

Bug reference:      18799
Logged by:          Daniel Elishakov
Email address:      dan-eli@mail.ru
PostgreSQL version: 16.6
Operating system:   ubuntu 20.04
Description:

Pointer, returned from function 'localtime' at timestamp.c:141, may be NULL
and is dereferenced at timestamp.c:142.In practice this shouldn't be a
problem, but for strictness it's better to fix this. See
'src/bin/initdb/findtimezone.c' for example of proper handling.