Fix: AudioStream._codec_context.coded_frame can be None, causing SEGV

Created by: radek-senfeld

Hi, I ran into this issue when trying to encode files with audio track. Took the same approach as you did in VideoStream.encode.

I'm wondering if coded_frame will ever be set in case of audio frames. Seems like coded_frame is referenced by video/image codecs only..

http://ffmpeg.org/doxygen/trunk/structAVCodecContext.html#afdebc347b4f74e0b9271ff37cabc96e8