Fix null conversion warning

On OS X compiling generates a warning:

```
andrew@humming:~/tmp/rtmidi% make
g++ -O3 -Wall -Iinclude -fPIC -INONE/include  -D__MACOSX_CORE__ -c RtMidi.cpp -o RtMidi.o
RtMidi.cpp:656:26: warning: implicit conversion of NULL constant to 'MIDIEntityRef' (aka 'unsigned int') [-Wnull-conversion]
  MIDIEntityRef entity = NULL;
                ~~~~~~   ^~~~
                         0
1 warning generated.
```