UNDERTOW-1332 Fix NPE in HttpServletRequestImpl#getLocalAddr

[aldaris]

The fix is very similar to what has been applied with UNDERTOW-108.

[undertow-pull-request]

Windows Build 2421 outcome was FAILURE using a merge of 8c5cd5e
Summary: Tests failed: 1 (1 new), passed: 3036, ignored: 690 Build time: 00:23:53

Failed tests

io.undertow.server.handlers.HttpContinueAcceptingHandlerTestCase.testHttpContinueRejected[proxy][http2-clear-upgrade]: java.lang.AssertionError: expected:<417> but was:<200>
	at io.undertow.server.handlers.HttpContinueAcceptingHandlerTestCase.testHttpContinueRejected(HttpContinueAcceptingHandlerTestCase.java:104)
------- Stderr: -------
13:24:38,323 DEBUG (XNIO-1 I/O-4) [io.undertow.server.handlers.proxy] <ProxyHandler.java:549> Sending request ClientRequest{path='/path', method=POST, protocol=HTTP/1.1} to target localhost/127.0.0.1:8888 for exchange HttpServerExchange{ POST /path request {Connection=[Keep-Alive], Expect=[100-continue], Content-Length=[16], Content-Type=[text/plain; charset=ISO-8859-1], Host=[localhost:7777]} response {}}
13:24:38,325 DEBUG (XNIO-1 I/O-4) [io.undertow.server.handlers.proxy] <ProxyHandler.java:566> Sent request ClientRequest{path='/path', method=POST, protocol=HTTP/1.1} to target 127.0.0.1 for exchange HttpServerExchange{ POST /path request {Connection=[Keep-Alive], Expect=[100-continue], Content-Length=[16], Content-Type=[text/plain; charset=ISO-8859-1], Host=[localhost:7777]} response {}}
13:24:38,326 DEBUG (XNIO-1 I/O-4) [io.undertow.server.handlers.proxy] <ProxyHandler.java:576> Received continue response to request ClientRequest{path='/path', method=POST, protocol=HTTP/1.1} to target localhost/127.0.0.1:8888 for exchange HttpServerExchange{ POST /path request {Connection=[Keep-Alive], Expect=[100-continue], Content-Length=[16], Content-Type=[text/plain; charset=ISO-8859-1], Host=[localhost:7777]} response {}}
13:24:38,327 DEBUG (XNIO-1 I/O-4) [io.undertow.client.http.HttpClientExchange] <HttpClientExchange.java:87> request terminated for request to localhost/127.0.0.1:8888 /path
13:24:38,328 DEBUG (XNIO-1 I/O-4) [io.undertow.client.http.HttpClientConnection] <HttpClientConnection.java:447> connection to localhost/127.0.0.1:8888 is being upgraded
13:24:38,328 DEBUG (XNIO-1 I/O-4) [io.undertow.server.handlers.proxy] <ProxyHandler.java:691> Received response ClientResponse{responseHeaders={content-length=[16], date=[Wed, 11 Apr 2018 20:23:00 GMT]}, responseCode=200, status='', protocol=HTTP/2.0} for request ClientRequest{path='/path', method=POST, protocol=HTTP/1.1} for exchange HttpServerExchange{ POST /path request {Connection=[Keep-Alive], Expect=[100-continue], Content-Length=[16], Content-Type=[text/plain; charset=ISO-8859-1], Host=[localhost:7777]} response {}}