Added a lock while creating a new session

gexec/session.go

diff --git a/gexec/session.go b/gexec/session.go
index 6d6ad34..387a72c 100644
--- a/gexec/session.go
+++ b/gexec/session.go
@@ -92,6 +92,8 @@
 	err := command.Start()
 	if err == nil {
 		go session.monitorForExit(exited)
+		trackedSessionsMutex.Lock()
+		defer trackedSessionsMutex.Unlock()
 		trackedSessions = append(trackedSessions, session)
 	}