HTTP/2: socket leak with "return 444" in error_page (ticket #2455).

HTTP/2: socket leak with "return 444" in error_page (ticket #2455).

Similarly to ticket #274 (7354:1812f1d79d84), early request finalization
without calling ngx_http_run_posted_requests() resulted in a connection
hang (a socket leak) if the 400 (Bad Request) error was generated in
ngx_http_v2_state_process_header() due to invalid request headers and
"return 444" was used in error_page 400.