Fix "unpack-objects --strict"

Fix "unpack-objects --strict"

When unpack-objects is run under the --strict option, objects that have
pointers to other objects are verified for the reachability at the end, by
calling check_object() on each of them, and letting check_object to walk
the reachable objects from them using fsck_walk() recursively.

The function however misunderstands the semantics of fsck_walk() function
when it makes a call to it, setting itself as the callback.  fsck_walk()
expects the callback function to return a non-zero value to signal an
error (negative value causes an immediate abort, positive value is still
an error but allows further checks on sibling objects) and return zero to
signal a success.  The function however returned 1 on some non error
cases, and to cover up this mistake, complained only when fsck_walk() did
not detect any error.

To fix this double-bug, make the function return zero on all success
cases, and also check for non-zero return from fsck_walk() for an error.

Signed-off-by: Junio C Hamano <gitster@pobox.com>