Avoid invalid length when building name symbol (#348)

[albanpeignier]

Small fix to avoid shorten element name or error on last closed not opened (aka orphan) element reported into ohler55/ox#348.

• Use strlen instead of calculation on dr->buf to manage both strings with and without \0 last character.
• Add tests on last element closed not opened (aka orphan).

Fixes ohler55/ox#348