Problem

c->pipeline is set to 1 in ngx_http_set_keepalive() when a
pipelined request is present in the connection buffer, and it is read in
ngx_http_finalize_connection() as one of the conditions that forces a
lingering close.

The flag is never cleared when a new request starts on a kept-alive
connection (ngx_http_keepalive_handler() resets c->sent and
c->destroyed, but not c->pipeline). As a result the flag can persist
across the keepalive boundary and trigger an unnecessary lingering close
for a later request that was not actually pipelined.

Fix

Reset c->pipeline = 0 in ngx_http_keepalive_handler() alongside the
existing c->sent/c->destroyed resets, restoring the flag's intended
present-tense meaning.

Impact is limited to a needless lingering close (no memory-safety or
security effect); this is a correctness cleanup.

Testing

Builds cleanly (--with-http_ssl_module --with-stream --with-stream_ssl_module --with-debug); no new warnings.