* mk/maint-parse-careful:
  peel_onion: handle NULL
  check return value from parse_commit() in various functions
  parse_commit: don't fail, if object is NULL
  revision.c: handle tag->tagged == NULL
  reachable.c::process_tree/blob: check for NULL
  process_tag: handle tag->tagged == NULL
  check results of parse_commit in merge_bases
  list-objects.c::process_tree/blob: check for NULL
  reachable.c::add_one_tree: handle NULL from lookup_tree
  mark_blob/tree_uninteresting: check for NULL
  get_sha1_oneline: check return value of parse_object
  read_object_with_reference: don't read beyond the buffer