Don't access line[-1] for a zero-length "line" from fgets.

Don't access line[-1] for a zero-length "line" from fgets.

A NUL byte at beginning of file, or just after a newline
would provoke an invalid buf[-1] access in a few places.

* builtin-grep.c (cmd_grep): Don't access buf[-1].
* builtin-pack-objects.c (get_object_list): Likewise.
* builtin-rev-list.c (read_revisions_from_stdin): Likewise.
* bundle.c (read_bundle_header): Likewise.
* server-info.c (read_pack_info_file): Likewise.
* transport.c (insert_packed_refs): Likewise.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>