[FAB-4497] Fix panic in TestEndedGoroutines

This fixes a possible panic in the test, that is
caused by a missing length check

Change-Id: I3afd31aa4ace3ab2237b192ec9ee4090bca77d12
Signed-off-by: Yacov Manevich <yacovm@il.ibm.com>

gossip/gossip/gossip_test.go

@@ -1114,6 +1114,9 @@ var clientConn = func(g goroutine) bool {
 }
 
 var testingg = func(g goroutine) bool {
+	if len(g.stack) == 0 {
+		return false
+	}
 	return strings.Index(g.stack[len(g.stack)-1], "testing.go") != -1
 }