pgsql: Revert ECPG's use of pnstrdup()

Revert ECPG's use of pnstrdup()

Commit 0b9466fce added a dependency on fe_memutils' pnstrdup() inside
informix.c. This adds an exit() path in a library, which we don't
want. (Unlike libpq, the ecpg libraries don't have an automated check
for that, but it makes sense to keep them to a similar standard.) The
ecpg code can already handle failure results from the *strdup() call
by itself.

Author: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Discussion: https://www.postgresql.org/message-id/CAOYmi+=pg=W5L1h=3MEP_EB24jaBu2FyATrLXqQHGe7cpuvwyg@mail.gmail.com

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2de129b356bfde9a02269c174396add065ace260

Modified Files
--------------
src/interfaces/ecpg/compatlib/informix.c | 23 +++++++++++++++++++++--
1 file changed, 21 insertions(+), 2 deletions(-)