This is illegal according to the C standard, sec. 7.1.4.

"If an argument to a function has an invalid value (such as a value
outside the domain of the function, or a pointer outside the address
space of the program, or a null pointer, or a pointer to non-modifiable
storage when the corresponding parameter is not const-qualified) or a
type (after promotion) not expected by a function with variable number
of arguments, the behavior is undefined."

7.24.1(2): "Where an argument declared as size_t n specifies the length
of the array for a function, n can have the value zero […] pointer
arguments on such a call shall still have valid values, as described in
7.1.4."

See https://www.imperialviolet.org/2016/06/26/nonnull.html

Introduce multi-purpose data/len members in struct cmark_node. This
is mainly used to store literal text for inlines, code and HTML blocks.

Move the content strbuf for blocks from cmark_node to cmark_parser.
When finalizing nodes that allow inlines (paragraphs and headings),
detach the strbuf and store the block content in the node's data/len
members. Free the block content after processing inlines.

Reduces size of struct cmark_node by 8 bytes.

Use zero-terminated C strings and a separate length field instead of
cmark_chunks. Literal inline text will now be copied from the parent
block's content buffer, slowing the benchmark down by 10-15%.

The node struct never references memory of other nodes now, fixing #309.
Node accessors don't have to check for delayed creation of C strings,
so parsing and iterating all literals using the public API should
actually be faster than before.

Use zero-terminated C strings instead of cmark_chunks without storing
the length. This introduces a few additional strlen computations,
but overhead should be low.

Allows to reduce size of struct cmark_node later.

This modifies unescaping in houdini_html_u.c rather than
the entity handling in inlines.c.  Unlike the other,
this approach works also in e.g. link titles.

See commonmark/commonmark.js#177.

to conform with spec change.