fix error with servicecode rendering

src/ems.cpp

@@ -950,6 +950,7 @@ void _process_UBAMonitorFast(uint8_t type, uint8_t * data, uint8_t length) {
     // read the service code / installation status as appears on the display
     EMS_Boiler.serviceCodeChar[0] = char(data[18]); // ascii character 1
     EMS_Boiler.serviceCodeChar[1] = char(data[19]); // ascii character 2
+    EMS_Boiler.serviceCodeChar[2] = '\0';           // null terminate string
 
     // read error code
     EMS_Boiler.serviceCode = (data[20] << 8) + data[21];

src/ems.h

@@ -172,7 +172,7 @@ typedef struct {           // UBAParameterWW
     uint8_t  curBurnPow;         // Burner current power
     float    flameCurr;          // Flame current in micro amps
     float    sysPress;           // System pressure
-    char     serviceCodeChar[2]; // 2 character status/service code
+    char     serviceCodeChar[3]; // 2 character status/service code
     uint16_t serviceCode;        // error/service code
 
     // UBAMonitorSlow