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 <[email protected]>
Helped-by: René Scharfe <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

builtin/am.c

@@ -1272,7 +1272,9 @@ static void get_commit_info(struct am_state *state, struct commit *commit)
 	buffer = logmsg_reencode(commit, NULL, get_commit_output_encoding());
 
 	ident_line = find_commit_header(buffer, "author", &ident_len);
-
+	if (!ident_line)
+		die(_("missing author line in commit %s"),
+		      oid_to_hex(&commit->object.oid));
 	if (split_ident_line(&id, ident_line, ident_len) < 0)
 		die(_("invalid ident line: %.*s"), (int)ident_len, ident_line);