s3:registry: add safety check for return value of tdb_unpack to regdb_fetch_keys_inte...

s3:registry: add safety check for return value of tdb_unpack to regdb_fetch_keys_internal()

Prevents segfaults in some situations.

(For a non existent or empty record, we sometimes rely on the fetch operation
 to return dsize==0 and sometimes we rely on dptr==NULL.)

Michael