Check if driver handle is nil before execing

Defend against tr.getDriverHandle being nil.  Exec handler checks if
task is running, but it may be stopped between check and driver handler
fetching.

client/allocrunner/taskrunner/task_runner.go

@@ -1347,7 +1347,12 @@ func appendTaskEvent(state *structs.TaskState, event *structs.TaskEvent, capacit
 }
 
 func (tr *TaskRunner) TaskExecHandler() drivermanager.TaskExecHandler {
-	return tr.getDriverHandle().ExecStreaming
+	// Check it is running
+	handle := tr.getDriverHandle()
+	if handle == nil {
+		return nil
+	}
+	return handle.ExecStreaming
 }
 
 func (tr *TaskRunner) DriverCapabilities() (*drivers.Capabilities, error) {