Upstream: introduced ngx_http_upstream_ssl_handshake_handler(). This change reworks 13a5f4765887 to only run posted requests once, with nothing on stack. Running posted requests with other request functions on stack may result in use-after-free in case of errors, similar to the one reported in #788. To only run posted request once, a separate function was introduced to be used as ssl handshake handler in c->ssl->handler, ngx_http_upstream_ssl_handshake_handler(). The ngx_http_run_posted_requests() is only called in this function, and not in ngx_http_upstream_ssl_handshake() which may be called directly on stack. Additionaly, ngx_http_upstream_ssl_handshake_handler() now does appropriate debug logging of the current subrequest, similar to what is done in other event handlers.