for-each-ref: fix segfault in copy_email

for-each-ref: fix segfault in copy_email

You can trigger a segfault in git.git by doing:

  git for-each-ref --format='%(taggeremail)' refs/tags/v0.99

The v0.99 tag is special in that it contains no "tagger"
header.

The bug is obvious in copy_email, which carefully checks to
make sure the result of a strchr is non-NULL, but only after
already having used it to perform other work. The fix is to
move the check up.

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