[webrtc-pc] insertDTMF input validation steps are still out of order a bit

jan-ivar has just created a new issue for 
https://github.com/w3c/webrtc-pc:

== insertDTMF input validation steps are still out of order a bit ==
The way [it's written right 
now](https://rawgit.com/w3c/webrtc-pc/c594cd214f0d71ff3c75a28e16be5dd695da15a6/webrtc.html#dom-rtcdtmfsender-insertdtmf),
 it's possible to observably set the attributes *duration* and 
*interToneGap* to invalid values by passing in an empty tone buffer. 
We should move the steps that clamp these to legal values to happen 
before we can return.

Please view or discuss this issue at 
https://github.com/w3c/webrtc-pc/issues/836 using your GitHub account