pgsql: Fix unportable code in SockAddr_cidr_mask: you can't assume that

Log Message:
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Fix unportable code in SockAddr_cidr_mask: you can't assume that
shifting left by full word width gives zero. Per bug report from
Tyson Thomson.

Modified Files:
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pgsql/src/backend/libpq:
ip.c (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/ip.c.diff?r1=1.29&r2=1.30)