mailinfo: avoid violating strbuf assertion

mailinfo: avoid violating strbuf assertion

In handle_from, we calculate the end boundary of a section
to remove from a strbuf using strcspn like this:

  el = strcspn(buf, set_of_end_boundaries);
  strbuf_remove(&sb, start, el + 1);

This works fine if "el" is the offset of the boundary
character, meaning we remove up to and including that
character. But if the end boundary didn't match (that is, we
hit the end of the string as the boundary instead) then we
want just "el". Asking for "el+1" caught an out-of-bounds
assertion in the strbuf library.

This manifested itself when we got a 'From' header that had
just an email address with nothing else in it (the end of
the string was the end of the address, rather than, e.g., a
trailing '>' character), causing git-mailinfo to barf.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>