SPDY: detect premature closing of stream.

The SPDY/3.1 specification requires that the server must respond with
a 400 "Bad request" error if the sum of the data frame payload lengths
does not equal the size of the Content-Length header.

This also fixes "zero size buf in output" alert, that might be triggered
if client sends a greater than zero Content-Length header and closes
stream using the FIN flag with an empty request body.