prevent flickering during OTA

I've almost toasted my wiring due to uncontrolled LED flickering during OTA.
So here is the fix - it prevents any strip updates when OTA is ongoing.

wled00/wled_server.cpp

@@ -336,6 +336,8 @@ void initServer()
     if(!index){
       DEBUG_PRINTLN(F("OTA Update Start"));
       WLED::instance().disableWatchdog();
+      OTAisRunning = true; // WLEDMM flicker fixer
+      strip.fill(BLACK);
       usermods.onUpdateBegin(true); // notify usermods that update is about to begin (some may require task de-init)
       lastEditTime = millis(); // make sure PIN does not lock during update
       #ifdef ESP8266
@@ -352,6 +354,7 @@ void initServer()
         usermods.onUpdateBegin(false); // notify usermods that update has failed (some may require task init)
         WLED::instance().enableWatchdog();
       }
+      OTAisRunning = false; // WLEDMM flicker fixer
     }
   });
 #else