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.

Modified Files:
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pgsql/src/backend/libpq:
hba.c (r1.140 -> r1.141)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c.diff?r1=1.140&r2=1.141)