PR go/65785

	PR go/65785
    net: don't run multicast listen test on nil interface in short mode

    This is a backport of https://golang.org/cl/17154.

    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.

    Reviewed-on: https://go-review.googlesource.com/17158

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@230695 138bc75d-0d04-0410-961f-82ee72b054a4