A special last buffer with cl->buf->pos set to NULL can be present in
a chain when writing request body if chunked encoding was used.  This
resulted in a NULL pointer dereference if it happened to be the only
buffer left after a do...while loop iteration in ngx_write_chain_to_file().

The problem originally appeared in nginx 1.3.9 with chunked encoding
support.  Additionally, rev. 3832b608dc8d (nginx 1.9.13) changed the
minimum number of buffers to trigger this from IOV_MAX (typically 1024)
to NGX_IOVS_PREALLOCATE (typically 64).

Fix is to skip such buffers in ngx_chain_to_iovec(), much like it is
done in other places.

The WINDOW_UPDATE frame could be left in the output queue for an indefinite
period of time resulting in the request timeout.

This might happen if reading of the body was triggered by an event unrelated
to client connection, e.g. by the limit_req timer.