re PR go/65785 (libgo TestIPv4MulticastListener test fails on machine with no network connection)

	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

gcc/go/gofrontend/MERGE

-b839c8c35af49bd6d86306ad34449654a4657cb1
+81dcb1ba4de82a6c9325cb322d5a832a6b1f168d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/net/listen_test.go

@@ -542,7 +542,7 @@ func TestIPv4MulticastListener(t *testing.T) {
 		// routing stuff for finding out an appropriate
 		// nexthop containing both network and link layer
 		// adjacencies.
-		if ifi == nil && !*testExternal {
+		if ifi == nil && (testing.Short() || !*testExternal) {
 			continue
 		}
 		for _, tt := range ipv4MulticastListenerTests {
@@ -618,7 +618,7 @@ func TestIPv6MulticastListener(t *testing.T) {
 		// routing stuff for finding out an appropriate
 		// nexthop containing both network and link layer
 		// adjacencies.
-		if ifi == nil && (!*testExternal || !*testIPv6) {
+		if ifi == nil && (testing.Short() || !*testExternal || !*testIPv6) {
 			continue
 		}
 		for _, tt := range ipv6MulticastListenerTests {