MINOR: http-htx: add BUG_ON to prevent API error on http_cookie_register

MINOR: http-htx: add BUG_ON to prevent API error on http_cookie_register

http_cookie_register() must be called on first invocation with the last
two arguments pointing both to a negative value. After it, they will be
updated to a valid index.

We must never have only the last argument as NULL as this will cause an
invalid array addressing. To clarify this a BUG_ON statement is
introduced.

This is linked to github issue #1967.