pgsql: Add missing bad-PGconn guards in libpq entry points.

Add missing bad-PGconn guards in libpq entry points.

There's a convention that externally-visible libpq functions should
check for a NULL PGconn pointer, and fail gracefully instead of
crashing.  PQflush() and PQisnonblocking() didn't get that memo
though.  Also add a similar check to PQdefaultSSLKeyPassHook_OpenSSL;
while it's not clear that ordinary usage could reach that with a
null conn pointer, it's cheap enough to check, so let's be consistent.

Daniele Varrazzo and Tom Lane

Discussion: https://postgr.es/m/CA+mi_8Zm_mVVyW1iNFgyMd9Oh0Nv8-F+7Y3-BqwMgTMHuo_h2Q@mail.gmail.com

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c19024d74e98e5c0ad95643ce584c52954d01b70

Modified Files
--------------
src/interfaces/libpq/fe-exec.c | 4 ++++
1 file changed, 4 insertions(+)