[grpc/exec] Do not refinc on a nil parent

When handling kthreads and we cannot find the parent, we print a
warning, but we also call refinc on a nil parent which results in a
panic in some cases.

This patch fixes that by avoiding the call of refinc in the case where
we cannot find a kthread's parent.

FIXES: #2614

Signed-off-by: Anastasios Papagiannis <[email protected]>

pkg/grpc/exec/kthread_init.go

@@ -23,6 +23,7 @@ func (msg *MsgKThreadInitUnix) HandleMessage() *tetragon.GetEventsResponse {
 	parent, err := process.Get(proc.UnsafeGetProcess().ParentExecId)
 	if err != nil {
 		logger.GetLogger().Warnf("Failed to find parent for kernel thread %d", msg.Unix.Msg.Parent.Pid)
+		return nil
 	}
 	parent.RefInc()
 	return nil