BUG/MINOR: http_ana: Be sure to have an allocated buffer to generate an error

BUG/MINOR: http_ana: Be sure to have an allocated buffer to generate an error

In http_reply_and_close() and http_server_error(), we must be sure to have an
allocated buffer (buf.size > 0) to consider it as a valid HTX message. For now,
there is no way to hit this bug. But a fix to support "empty" error messages in
HTX is pending. Such empty messages, after parsing, will be converted into
unallocated buffer (buf.size == 0).

This patch must be backported to 2.0 and 1.9. owever, the patch will have to be
adapted.