Avoid NPEs in JavaNetAuthenticator

[DanielThomas]

Configuring JavaNetAuthenticator as the proxyAuthenticator when using the default ProxySelector causes NPEs to be thrown because the socket addresses returned are unresolved:

Exception in thread "main" java.lang.NullPointerException: address() as InetSocketAddress).address must not be null
	at okhttp3.internal.authenticator.JavaNetAuthenticator.connectToInetAddress(JavaNetAuthenticator.kt:91)
	at okhttp3.internal.authenticator.JavaNetAuthenticator.authenticate(JavaNetAuthenticator.kt:53)
	at okhttp3.internal.http.RetryAndFollowUpInterceptor.followUpRequest(RetryAndFollowUpInterceptor.kt:220)
	at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.kt:108)
	at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.kt:109)
	at okhttp3.internal.connection.RealCall.getResponseWithInterceptorChain$okhttp(RealCall.kt:201)
	at okhttp3.internal.connection.RealCall.execute(RealCall.kt:154)

The implementation could also NPE on a 407 from an non-proxy host, so I've handled this too.