shortlog: fix segfault on empty authorname

shortlog: fix segfault on empty authorname

The old code looked backwards from the email address to parse the name,
allowing an arbitrary number of spaces between the two. However, in the case
of no name, we looked back too far to the 'author' (or 'Author:') header.
Instead, remove at most one space between name and address.

The bug was triggered by commit febf7ea4bed from linux-2.6.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>