cls/cas/cls_cas_internal: Initialize 'hash' value before decoding into it

cls/cas/cls_cas_internal: Initialize 'hash' value before decoding into it

In the decode function for chunk_refs_by_hash_t, initialize the variable
'hash' of type ceph_le32 to zero before its first use.
This prevents the variable from containing dirty (uninitialized) values,
which could lead to unexpected behavior later in the code.

Fixes: https://tracker.ceph.com/issues/64854
Signed-off-by: Nitzan Mordechai <nmordech@redhat.com>