Add require statements to ERC777 internal functions

[nventuro]

#2027 made some functions, including _approve, internal. A comment indicated that doing this required that a commented-out require statement be restored, but this was not done.

This leads to some of the arguments of these internal functions not being properly validated, including allowing for _approve's holder to be the zero address. We should:

1. restore this require statement
2. evaluate which arguments of the other functions made internal in that PR should be validated
3. and backport the fix to v2.5