bug fix for LabelBMFont::getBoundingBox()

`LabelBMFont::getBoundingBox()` is the bounding box of LabelBMFont relative to its parent.

while `_label->getBoundingBox()` is the bounding box of _label relative to LabelBMFont.

Although the size of the above 2 boxes are the same, their offset are not necessarily the same.

(`_label` is always positioned at `Point::ZERO` of `LabelBMFont`, which is not necessarily the case for `LabelBMFont`)

This bug can cause that we can't correctly detect whether a `LabelBMFont` is clicked or not:http://www.cocoachina.com/bbs/read.php?tid=209406
Thank you for report this isue.
Please consult https://github.com/cocos2d/cocos2d-x/pull/7622/files for fix this issue.