Use ENCTYPE_NULL in the krb5_crypto_init() call.

That appears to be a name supplied both by MIT and Heimdal Kerberos.
Using it makes it a bit clearer what the code is doing, and might avoid
type clash warnings if it's the right type (e.g., if it's a member of an
enum, as it is in Heimdal, and the corresponding argument to
krb5_crypto_init() is of the same type, the types will match).

Change-Id: I81b79223f789b8d1ec47180b7636ac1d83e03681
Reviewed-on: https://code.wireshark.org/review/5898
Reviewed-by: Guy Harris <guy@alum.mit.edu>