pgsql: Add comment explaining possible compiler warning: /* *

Log Message:
-----------
Add comment explaining possible compiler warning:

/*
* Some compilers with throw a warning knowing this test can never be
* true because off_t can't exceed the compared maximum.
*/
if (th->fileLen > MAX_TAR_MEMBER_FILELEN)
die_horribly(AH, modulename, "archive member too large for tar format\n");

Modified Files:
--------------
pgsql/src/bin/pg_dump:
pg_backup_tar.c (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_tar.c.diff?r1=1.45&r2=1.46)