bugfix : use streamer as weak ref

umeng 上看到的crash, 应该是野指针了

** -[DOUAudioStreamer setStatus:]: unrecognized selector sent to instance 0x156a7370 **
xcrun atos -o DoubanRadio -arch armv7 0x13c78f
-[DOUAudioEventLoop _handleEvent:withStreamer:] (in DoubanRadio) + 1007
-[DOUAudioStreamer setStatus:]: unrecognized selector sent to instance 0x156a7370