fix isAvailable. Returned true at startup, though still no message were received by inverter

src/hm/hmInverter.h

@@ -391,6 +391,10 @@ class Inverter {
 
         bool isAvailable() {
             bool avail = false;
+
+            if (!((recordMeas.ts > 0) || (recordInfo.ts > 0) || (recordConfig.ts > 0) || (recordAlarm.ts > 0)))
+                return avail;
+
             if((*timestamp - recordMeas.ts) < INVERTER_INACT_THRES_SEC)
                 avail = true;
             if((*timestamp - recordInfo.ts) < INVERTER_INACT_THRES_SEC)