only check line after Contributors if it exists

[sbfnk]

Fixes #40

Also combines the two checks into one as they have the same outcome.

[mpadge]

Thanks so much @sbfnk. I was just working on solution myself. I think it's safer just to wrap the current lines:
https://github.com/ropenscilabs/allcontributors/blob/d9ce29682297494e40f26e3a257967a08ef2add3/R/add-contributors.R#L333-L338

in a single if (contribs_sec < length (x)) {...}. Could you please make that modification? Then I'll happily merge. Thanks for improving the package!

[sbfnk]

OK -- done!