pgsql: Fix case where "PM" to_timestamp() mask was eating too many

Log Message:
-----------
Fix case where "PM" to_timestamp() mask was eating too many characters.
Report from Josh Tolley.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
formatting.c (r1.110 -> r1.111)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c.diff?r1=1.110&r2=1.111)