sysusers: make sure to reset the returned value when EOF is reached in fget*ent_sane...

sysusers: make sure to reset the returned value when EOF is reached in fget*ent_sane() wrappers (#8737)

To indicate that the there're no more entries, these wrappers return false but
did leave the passed pointed unmodified.

However EOF is not an error and is a very common case so initialize the output
argument to NULL even in this case so callers don't need to do that.

Fixes: #8721