return after decisionHandler called to avoid call it more than once

[wrlqwe]

in the latest version in [WKWebViewJavascriptBridge webView:decidePolicyForNavigationAction:decisionHandler:], after decisionHander executed:

    if ([_base isWebViewJavascriptBridgeURL:url]) {
        if ([_base isBridgeLoadedURL:url]) {
            [_base injectJavascriptFile];
        } else if ([_base isQueueMessageURL:url]) {
            [self WKFlushMessageQueue];
        } else {
            [_base logUnkownMessage:url];
        }
        decisionHandler(WKNavigationActionPolicyCancel);
    }

decisionHandler passed to _webViewDelegate and the delegate method may call decisionHandler again, most conditions it work well, but it leads crash on my iOS11+XCode9 beta environment.
In my view, a decision cannot be made for twice, the decisionHandler that consumed should not pass out side of this method, I hope this commit can fix it.

[wrlqwe]

please review my code, I'm not sure this commit meets the requirements for repo users or is it good enough...

[marcuswestin]

Thank you!