Check for callback in makeAsyncHandler (#395)

This PR fixes regression from 3540972 and adds check for callback validity before putting it into event loop for sendAsync calls.

geth/jail/handlers.go

@@ -75,10 +75,12 @@ func makeAsyncSendHandler(jail *Jail, cellInt common.JailCell) func(call otto.Fu
 		go func() {
 			response := jail.Send(call)
 
-			// run callback asyncronously with args (error, response)
 			callback := call.Argument(1)
+			if callback.Class() == "Function" {
+				// run callback asyncronously with args (error, response)
 				err := otto.NullValue()
 				cell.CallAsync(callback, err, response)
+			}
 		}()
 		return otto.UndefinedValue()
 	}