Out of range panic fix

[artemseleznev]

The Svace static analyzer (https://www.ispras.ru/en/technologies/svace/) flagged this code as suspicious: it could lead to a panic (out of range) if len(out) == 0. It might happen if text would be e.g. "bla:bla"

[SuperQ]

It would also be useful to add some tests.

[discordianfish]

Yeah I feel we should error instead