fix bug:  export frame isn't start as 0 time.
fix bug:  export frame isn't start as 0 time.

cocos2d-x/fbx-conv,lvlonggame/fbx-conv,super626/fbx-conv,super626/fbx-conv,lvlonggame/fbx-conv,lvlonggame/fbx-conv,cocos2d-x/fbx-conv,super626/fbx-conv,cocos2d-x/fbx-conv