DiskLoc56Bit::Z accesses bytes not owned by current DiskLoc56Bit object

In https://github.com/mongodb/mongo/blob/57213bdd0d1abf214f4a7db3f6e4aa161a69a9c5/src/mongo/db/btree.h#L194

DiskLoc56Bit::Z does an 8 byte read from a reinterpret_cast'ed this pointer. However, a DiskLoc56Bit is only 7 bytes long, so this is undefined behavior.

The 'Z' method should be rewritten to only access the bytes owned by the current DiskLoc56Bit.