packed_object_info(): convert to new revindex API

packed_object_info(): convert to new revindex API

Convert another call of 'find_pack_revindex()' to its replacement
'pack_pos_to_offset()'. Likewise:

  - Avoid manipulating `struct packed_git`'s `revindex` pointer directly
    by removing the pointer-as-array indexing.

  - Add an additional guard to check that the offset 'obj_offset()'
    points to a real object. This should be the case with well-behaved
    callers to 'packed_object_info()', but isn't guarenteed.

    Other blocks that fill in various other values from the 'struct
    object_info' request handle bad inputs by setting the type to
    'OBJ_BAD' and jumping to 'out'. Do the same when given a bad offset
    here.

    The previous code would have segfaulted when given a bad
    'obj_offset' value, since 'find_pack_revindex()' would return
    'NULL', and then the line that fills 'oi->disk_sizep' would try to
    access 'NULL[1]' with a stride of 16 bytes (the width of 'struct
    revindex_entry)'.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>