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.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index ba21c01..577c2b5 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1591,7 +1591,7 @@
 
         if (rc == NGX_AGAIN) {
 
-            if (u->buffer.pos == u->buffer.end) {
+            if (u->buffer.last == u->buffer.end) {
                 ngx_log_error(NGX_LOG_ERR, c->log, 0,
                               "upstream sent too big header");