ecpg: Fix NULL pointer dereference during connection lookup

ecpg: Fix NULL pointer dereference during connection lookup

ECPGconnect() caches established connections to the server, supporting
the case of a NULL connection name when a database name is not specified
by its caller.

A follow-up call to ECPGget_PGconn() to get an established connection
from the cached set with a non-NULL name could cause a NULL pointer
dereference if a NULL connection was listed in the cache and checked for
a match.  At least two connections are necessary to reproduce the issue:
one with a NULL name and one with a non-NULL name.

Author:  Aleksander Alekseev <aleksander@tigerdata.com>
Discussion: https://postgr.es/m/CAJ7c6TNvFTPUTZQuNAoqgzaSGz-iM4XR61D7vEj5PsQXwg2RyA@mail.gmail.com
Backpatch-through: 13