net: fix a bad cast in dnsmsg.go
net: fix a bad cast in dnsmsg.go

Incorrect cast was causing panics when
calling String() on dnsMsg with dnsRR_A
answers.

R=golang-dev, dave, rsc
CC=golang-dev
http://codereview.appspot.com/6818043

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang