Upstream: fixed "too big header" check. If header filter postponed processing of a header by returning NGX_AGAIN and not moved u->buffer->pos, previous check incorrectly assumed there is additional space and did another recv() with zero-size buffer. This resulted in "upstream prematurely closed connection" error instead of correct "upstream sent too big header" one. Patch by Feibo Li.

# Copyright (C) Igor Sysoev
# Copyright (C) Nginx, Inc.


have=NGX_SUPPRESS_WARN . auto/have

have=NGX_SMP . auto/have