ot::Spinel::Buffer::OutFrameGetLength() method

Returns the number of bytes (length) of current or front frame in the NCP frame buffer. If there is an active output from being read (an output frame was prepared earlier with successful call to `OutFrameBegin()`), this method returns the length of the current output frame. If there is no current active frame, the length of the front frame in the queue (the next frame which would have been read) will be returned. If there is no frame in buffer, this method returns zero.