Don't use $author_name undefined when $from contains no /\s</.

Don't use $author_name undefined when $from contains no /\s</.

I noticed a case not handled in a recent patch.
Demonstrate it like this:

  $ touch new-file
  $ git-send-email --dry-run --from j --to k new-file 2>err
  new-file
  OK. Log says:
  Date: Thu, 19 Oct 2006 10:26:24 +0200
  Sendmail: /usr/sbin/sendmail
  From: j
  Subject:
  Cc:
  To: k

  Result: OK
  $ cat err
  Use of uninitialized value in pattern match (m//) at /p/bin/git-send-email line 416.
  Use of uninitialized value in concatenation (.) or string at /p/bin/git-send-email line 420.
  Use of uninitialized value in concatenation (.) or string at /p/bin/git-send-email line 468.

There's a patch for the $author_name part below.

The example above shows that $subject may also be used uninitialized.
That should be easy to fix, too.

Signed-off-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>