The gccgo bug report https://gcc.gnu.org/PR65785 points out that the
multicast listen tests will use the network even with -test.short.
Fix test by checking testing.Short with a nil interface.

Change-Id: I7eab8df34fe3b78fc376912312fac9d0f94977f1
Reviewed-on: https://go-review.googlesource.com/17154Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>