cmMidiPort.c: Use 0xff as invalid status byte instead of -1 in cmMpParseMidiData()

and cmMpParserMidiTriple() to eliminate 'always true' warning.

cmMidiPort.c

 {
   cmMpRC_t rc = kOkMpRC;
   cmMpParser_t* p = _cmMpParserFromHandle(h);
-  cmMidiByte_t mb = -1;
+  cmMidiByte_t mb = 0xff; // a midi triple may never have a status of 0xff
 
   if( d0 == 0xff )
     p->dataCnt = 0;
       break;
   }
 
-  if( mb != -1 )
+  if( mb != 0xff )
     _cmMpParserStoreChMsg(p,deltaMicroSecs,mb);
   
   p->dataCnt = cmInvalidCnt;