QUIC: fixed stream cleanup (ticket #2586).

QUIC: fixed stream cleanup (ticket #2586).

Stream connection cleanup handler ngx_quic_stream_cleanup_handler() calls
ngx_quic_shutdown_stream() after which it resets the pointer from quic stream
to the connection (sc->connection = NULL).  Previously if this call failed,
sc->connection retained the old value, while the connection was freed by the
application code.  This resulted later in a second attempt to close the freed
connection, which lead to allocator double free error.

The fix is to reset the sc->connection pointer in case of error.