Set null terminator in cmark::html::reset()
Set null terminator in cmark::html::reset()

This fixes issue where parsing 0 length html strings would return a html
object with size == 0 but no null terminator set a data()[0] screwing
with the escaping code in write_javascript

JaySandhu/vimdown,JaySandhu/vimdown,JaySandhu/vimdown