split: reset errno prior to getline()

Something else may have set errno, breaking the post-getline() logic
that tries to detect the getline() error. This was initially noted in
a jail on a system that has HPET, in a jail that does not expose
/dev/hpet0 -- we see an earlier error in libc's vdso bits.

Fixes: 5c053aa3c5e90 ("split: switch to getline() [...]")