SPDY: fixed possible segfault.

While processing a DATA frame, the link to related stream is stored in spdy
connection object as part of connection state.  But this stream can be closed
between receiving parts of the frame.

src/http/ngx_http_spdy.c

@@ -2626,6 +2626,10 @@ ngx_http_spdy_close_stream(ngx_http_spdy_stream_t *stream, ngx_int_t rc)
         }
     }
 
+    if (sc->stream == stream) {
+        sc->stream = NULL;
+    }
+
     sscf = ngx_http_get_module_srv_conf(sc->http_connection->conf_ctx,
                                         ngx_http_spdy_module);