Protect against setting file attributes with invalid cipher
Protect against setting file attributes with invalid cipher

In case the app attempts to set a file attribute and the nodekey is not
available (cannot be decrypted), the cipher might be null, and `putfa()`
immediately dereference.
This commit will protect against that scenario.

meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk,meganz/sdk