rtaudio/RtAudio.cpp: check for null pointer

rtaudio/RtAudio.cpp: check for null pointer

Fixes Coverity CID 710858: Dereference after null check (FORWARD_NULL)
Comparing "this->rtapi_" to null implies that "this->rtapi_" might be null.
 146    if ( rtapi_ ) return;
...
Passing null variable "this->rtapi_" to function "RtApi::getDeviceCount()", which dereferences it. (The dereference happens because this is a virtual function call.)
 159    if ( rtapi_->getDeviceCount() ) break;