When parsing a header that has a continuation line, today we append that
line to it, which copies the whole header each time.
On normal messages this is negligible, but a header with a very large
number of continuation lines would cause a DoS through work
amplification.
This patch fixes the problem by making header continuation handling
linear: we now accumulate the header lines in a list, and then join them
only once.
Thanks to John JEAN <john@wargan.com> who reported a similar issue
(fixed in a previous patch), which led to finding this bug.
Alberto Bertogli