realtime mode bugfixes

- busses: ignore not valid bus instances
- "use main segment only": prevent flickering du to intermitted strip.service().

wled00/bus_manager.h

@@ -190,6 +190,7 @@ class Bus {
     inline        uint8_t getAutoWhiteMode()          const { return _autoWhiteMode; }
     inline static void    setGlobalAWMode(uint8_t m)  { if (m < 5) _gAWM = m; else _gAWM = AW_GLOBAL_DISABLED; }
     inline static uint8_t getGlobalAWMode()           { return _gAWM; }
+    inline        bool    isValid()                   const {return _valid;}
 
     inline static uint32_t restore_Color_Lossy(uint32_t c, uint8_t restoreBri) { // shamelessly grabbed from upstream, who grabbed from NPB, who ..
       if (restoreBri < 255) {