QUIC: fixed possible use-after-free on stream cleanup.

QUIC: fixed possible use-after-free on stream cleanup.

A QUIC stream could be destroyed by handler while in ngx_quic_stream_input().
To detect this, ngx_quic_find_stream() is used to check that it still exists.

Previously, a stream id was passed to this routine off the frame structure.
In case of stream cleanup, it is freed along with other frames belonging to
the stream on cleanup.  Then, a cleanup handler reuses last frames to update
MAX_STREAMS and serve other purpose.  Thus, ngx_quic_find_stream() is passed
a reused frame with zeroed out part pointed by stream_id.  If a stream with
id 0x0 still exists, this leads to use-after-free.