Fix null conversion warning

Avoid this compilation warning:

    warning: implicit conversion of NULL constant to 'MIDIEntityRef' (aka 'unsigned int') [-Wnull-conversion]

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