Fix null pointer as argument

[razzeee]

Fuzzing did show two cases of undefined behavior. memcpy can cause problems, so we'll use a loop instead.

Fixes https://github.com/elm-tooling/tree-sitter-elm/issues/89

[razzeee]

See https://stackoverflow.com/questions/5243012/is-it-guaranteed-to-be-safe-to-perform-memcpy0-0-0