We need to free what we have allocated.

Closes GH-6327.

We need to check whether the given `type`s and `encoding`s are within
bounds to avoid segfaults and out-of-bound reads.

Closes GH-6323.

We have to clean up even on failure.

Closes GH-6322.

Unless `topbod` is of `TYPEMULTIPART`, `mail_free_body()` does not free
the `nested.part`; while we could do this ourselves, instead we just
ignore additional bodies in this case, i.e. we don't attach them in the
first place.

Closes GH-6321.

We separate the input arrays and all sub-arrays to avoid modification
of the passed parameters.

This should be rewritten to use `zend_string`s for the "master" branch.

Closes GH-6316.

We have to cater to non-associative arrays where the key may be `NULL`;
we just skip these elements.

Closes GH-6315.

The documentation of `tidyNode::isHtml()` states that this method
"checks if a node is part of a HTML document".  That is, of course,
nonsense, since a tidyNode is "an HTML node in an HTML file, as
detected by tidy."

What this method is actually supposed to do is to check whether a node
is an element (unless it is the root element).  This has been broken by
commit d8eeb8e[1], which assumed that `enum TidyNodeType` would
represent flags of a bitmask, what it does not.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=d8eeb8e28673236bca3f066ded75037a5bdf6378>

Closes GH-6290.

There is no such thing as the "end of the unix epoch", and if it was,
it would certainly not be 2037-10-11T02:00:00.  There is, however,
potential integer overflow which we need to avoid.

Closes GH-6288.