use parse_commit_or_die instead of segfaulting

Some unchecked calls to parse_commit should obviously die on
error, because their next step is to start looking at the
parsed fields, which will cause a segfault. These are
obvious candidates for parse_commit_or_die, which will be a
strict improvement in behavior.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -796,7 +796,7 @@ static int switch_branches(const struct checkout_opts *opts,
 		new->commit = old.commit;
 		if (!new->commit)
 			die(_("You are on a branch yet to be born"));
-		parse_commit(new->commit);
+		parse_commit_or_die(new->commit);
 	}
 
 	ret = merge_working_tree(opts, &old, new, &writeout_error);
@@ -959,7 +959,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 		/* not a commit */
 		*source_tree = parse_tree_indirect(rev);
 	} else {
-		parse_commit(new->commit);
+		parse_commit_or_die(new->commit);
 		*source_tree = new->commit->tree;
 	}
 

builtin/fast-export.c

@@ -286,7 +286,7 @@ static void handle_commit(struct commit *commit, struct rev_info *rev)
 
 	rev->diffopt.output_format = DIFF_FORMAT_CALLBACK;
 
-	parse_commit(commit);
+	parse_commit_or_die(commit);
 	author = strstr(commit->buffer, "\nauthor ");
 	if (!author)
 		die ("Could not find author in commit %s",
@@ -307,7 +307,7 @@ static void handle_commit(struct commit *commit, struct rev_info *rev)
 	if (commit->parents &&
 	    get_object_mark(&commit->parents->item->object) != 0 &&
 	    !full_tree) {
-		parse_commit(commit->parents->item);
+		parse_commit_or_die(commit->parents->item);
 		diff_tree_sha1(commit->parents->item->tree->object.sha1,
 			       commit->tree->object.sha1, "", &rev->diffopt);
 	}