HTTP: fixed r.subrequest() error handling.

HTTP: fixed r.subrequest() error handling.

Previously, when at least 2 subrequests were scheduled they both
succeed, but the callback for the second threw an exception
heap-use-after-free happened: a nested chain of
ngx_http_run_posted_requests() calls and terminating request in the
inner call left outer calls with already freed request pointer.

The issue was introduced in 0.8.1 (4cb8e873e8c6).