Check for a NULL return value from read_author_ident() that indicates
an error. Previously the NULL author was passed to commit_tree() which
would then fallback to using the default author when creating the new
commit. This changed the date and potentially the author of the commit
which corrupted the author data compared to its expected value.
Helped-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>