pgsql: Remove compiler warning by casting SNPRINTF() call to void.

Log Message:
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Remove compiler warning by casting SNPRINTF() call to void.

Report from Gevik Babakhani.

Modified Files:
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pgsql/src/backend/utils/adt:
inet_net_ntop.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/inet_net_ntop.c.diff?r1=1.21&r2=1.22)