Fix a CFStringRef leak in RTMidi Port Name

In certain circumstances when building a compound port name
(for instance, with an IAC bus), the string math to concatenate
the bus and driver does not release one of the source strings,
causing a CFStringRef leak on macOS.

This fixes it by doing the appropriate CFRelease after string
concatenation.

RtMidi.cpp

@@ -1376,6 +1376,7 @@ CFStringRef CreateEndpointName( MIDIEndpointRef endpoint, bool isExternal )
           CFStringInsert( result, 0, CFSTR(" ") );
 
         CFStringInsert( result, 0, str );
+        CFRelease(str);
       }
     }
   }