Ensure the pack index is opened before access

Ensure the pack index is opened before access

In this particular location of fsck the index should have already
been opened by verify_pack, which is called just before we get
here and loop through the object names.  However, just in case a
future version of that function does not use the index file we'll
double-check its open before we access the num_objects field.

Better safe now than sorry later.

Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>