pgsql: Fix a potential backend crash during authentication when parsing

Log Message:
-----------
Fix a potential backend crash during authentication when parsing a
malformed ident map file. This was introduced by the linked list
rewrite in 8.0 -- mea maxima culpa.

Per Coverity static analysis performed by EnterpriseDB.

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
pgsql/src/backend/libpq:
hba.c (r1.136 -> r1.136.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c.diff?r1=1.136&r2=1.136.4.1)