Export: don't try to read past the document end

Export: don't try to read past the document end

A faulty bound checking resulted in reading a byte past the document
end, which resulted in Scintilla returning byte 0 because the position
was invalid.  By adding this NUL byte to the string used to build the
body, the body was truncated after the last document byte (as C strings
are NUL-terminated), leading to any format structure after it to be
missing from the output.

This broke HTML and LaTeX export if the last line didn't end with a
newline, as the last line's style closing structure were missing.