pgsql: Slightly more paranoia and slightly better comments for use of

Log Message:
-----------
Slightly more paranoia and slightly better comments for use of
Windows-specific MultiByteToWideChar/WideCharToMultiByte calls.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
oracle_compat.c (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/oracle_compat.c?r1=1.74&r2=1.75)