git-am: handle missing "author" when parsing commit

We try to parse the "author" line out of a commit buffer. We handle the case that split_ident_line() doesn't work, but we don't do any error checking that we found an "author" line in the first place! This would cause us to segfault on such a corrupt object. Let's put in an explicit NULL check (we can just die(), which is what a bogus split would do, too). As a bonus, this silences a warning when compiling with gcc 9.2.1 using "-flto -O3", which claims that ident_len may be uninitialized (it would only be if we had a NULL here). Reported-by: Stephan Beyer <s-beyer@gmx.net> Helped-by: René Scharfe <l.s.r@web.de> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>