SPDY: fixed segfault with "client_body_in_file_only" enabled.

It is possible to send FLAG_FIN in additional empty data frame, even if it is
known from the content-length header that request body is empty.  And Firefox
actually behaves like this (see ticket #357).

To simplify code we sacrificed our microoptimization that did not work right
due to missing check in the ngx_http_spdy_state_data() function for rb->buf
set to NULL.