Merge pull request #8 from drewish/fix-osx-warning

Fix null conversion warning

RtMidi.cpp

@@ -653,7 +653,7 @@ CFStringRef EndpointName( MIDIEndpointRef endpoint, bool isExternal )
     CFRelease( str );
   }
 
-  MIDIEntityRef entity = NULL;
+  MIDIEntityRef entity = 0;
   MIDIEndpointGetEntity( endpoint, &entity );
   if ( entity == 0 )
     // probably virtual