rtaudio/RtAudio.h: initialize variable

rtaudio/RtAudio.h: initialize variable

Fixes Coverity CID 710877: Uninitialized scalar field (UNINIT_CTOR)
Class member declaration for ""thread"".
535  ThreadHandle thread;
...
Non-static class member ""thread"" is not initialized in this constructor nor in any functions that it calls.
543    :object(0), callback(0), userData(0), apiInfo(0), isRunning(false) {}