pgsql: Fix off-by-one memory allocation, as reported by Rod Taylor.

Log Message:
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Fix off-by-one memory allocation, as reported by Rod Taylor. Also
avoid repalloc'ing twice when once is sufficient.

Tags:
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REL7_3_STABLE

Modified Files:
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pgsql/src/backend/libpq:
hba.c (r1.87.2.4 -> r1.87.2.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c.diff?r1=1.87.2.4&r2=1.87.2.5)