Clear errno before calling strtol() in spell.c.

Clear errno before calling strtol() in spell.c.

Per POSIX, a caller of strtol() that wishes to check for errors must
set errno to 0 beforehand.  Several places in spell.c neglected that,
so that they risked delivering a false overflow error in case errno
had been ERANGE already.  Given the lack of field reports, this case
may be unreachable at present --- but it's surely trouble waiting to
happen, so fix it.

Author: Jacob Brazeal <jacob.brazeal@gmail.com>
Discussion: https://postgr.es/m/CA+COZaBhsq6EromFm+knMJfzK6nTpG23zJ+K2=nfUQQXcj_xcQ@mail.gmail.com
Backpatch-through: 13