Fix race condition in DNS resolving.

If the querying thread was faster than the DNS resolving thread startup,
resolving would crash with the following stacktrace:

FATAL EXCEPTION: WebViewCoreThread
E/AndroidRuntime(7160): java.lang.NullPointerException
E/AndroidRuntime(7160):      at android.webkit.DnsResolver.resolveDnsForHostMap(DnsResolver.java:187)
E/AndroidRuntime(7160):      at android.webkit.BrowserFrame.startDnsPrefetch(BrowserFrame.java:248)
E/AndroidRuntime(7160):      at android.webkit.WebViewCore$EventHub$1.handleMessage(WebViewCore.java:1400)
E/AndroidRuntime(7160):      at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(7160):      at android.os.Looper.loop(Looper.java:130)
E/AndroidRuntime(7160):      at android.webkit.WebViewCore$WebCoreThread.run(WebViewCore.java:645)
E/AndroidRuntime(7160):      at java.lang.Thread.run(Thread.java:1019)