Fix NPE in ClientToProxyConnection while closing the client connection

[igor-dmitriev]

Fixes https://github.com/verygoodsecurity/vault/issues/1402

Due to currentHttpRequest can be null need to add not null verification, otherwise NPE can occurred.

https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ProxyToServerConnection.java#L290

The currentHttpRequest goes the following route:

1. https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ProxyToServerConnection.java#L519
2. https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ClientToProxyConnection.java#L481
3. https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ClientToProxyConnection.java#L820
4. https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ClientToProxyConnection.java#L822
5. As you can see in shouldCloseClientConnection when response is chunked not-null verification is present
   https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ClientToProxyConnection.java#L858

but DOES NOT in keep alive checking

https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/impl/ClientToProxyConnection.java#L869

that's why we got NPE

I've checked all other places where NPE can be raised as well, just need to fix it here.

ProxyToServerConnection

The current HTTP Request can be null when this proxy is
negotiating a CONNECT request with a chained proxy
while it is running as a MITM.

[mjallday]

👍 looks. can we get a test?

[igor-dmitriev]

@mjallday I couldn't reproduce the issue locally, unfortunately :(
I was trying to create a test to catch this NPE