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

From-SVN: r230695