Fixes bug in stopMusic()

In Chrome, it can take a few seconds for the `ended` property of music to become `false`. Therefore if one plays music, then immediately stops the music, `_stopAudio` can silently fail and then `stopMusic` mistakenly sets `_currMusic` to `null`, which leaves the music playing and no way to stop it. The problem is compounded if this situation is repeated. There can be many music tracks playing and it slows down the browser tremendously.

This pull request fixes the bug by verifying that `_stopAudio` worked.
Compatible with builder
Merge pull request #1798 from NatWeiss/patch-8

Fixes bug in stopMusic()
Merge pull request #1799 from VisualSJ/develop-3.0

Compatible with builder
Closed #4926: add a condition to check plist file in SpriteFrameCache.addSpriteFrames
Merge branch 'develop-3.0' of https://github.com/cocos2d/cocos2d-html5 into Iss4926_SpriteFrame
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-html5 into Iss4926_SpriteFrame
Fixed #4926: add a condition to support playing sound from 'file://' origin
Merge pull request #1801 from dingpinglv/Iss4926_SpriteFrame

Closed #4926: fixed some bugs and merge develop 3.0 to develop branch