HTTP/2: fixed null pointer dereference with server push. r->headers_in.host can be NULL in ngx_http_v2_push_resource(). This happens when a request is terminated with 400 before the :authority or Host header is parsed, and either pushing is enabled on the server{} level or error_page 400 redirects to a location with pushes configured. Found by Coverity (CID 1429156).