Kill MAX_BUF_LEN

Kill MAX_BUF_LEN

There's no real point in having a hard limit for struct oc_text_buf, the
whole point of which is that it is dynamically allocated. Just guard
against the int buf_len overflowing.

In process_http_response() the hard-coded buf[] array is only used for
headers one line at a time now, so 8KiB should suffice.

Fixes: #39
Signed-off-by: David Woodhouse <dwmw2@infradead.org>