Add isRunning getter to check is ::begin already called (#7219)

Author: Viktr

libraries/ESP8266mDNS/src/LEAmDNS.h

@@ -194,6 +194,11 @@ class MDNSResponder
     // for compatibility...
     bool setHostname(const String& p_strHostname);
 
+    bool isRunning(void)
+    {
+        return (m_pUDPContext != 0);
+    }
+
     /**
         hMDNSService (opaque handle to access the service)
     */