Fix garbage return in extract_cammacs()

If no cammacs were present, we would never initialize ret before
returning it.  (extract_cammcs() is currently never called with an
empty or null list, so this does not manifest as a bug.)

[ghudson@mit.edu: amended commit message]

src/lib/krb5/krb/authdata.c

@@ -546,7 +546,7 @@ static krb5_error_code
 extract_cammacs(krb5_context kcontext, krb5_authdata **cammacs,
                 const krb5_keyblock *key, krb5_authdata ***ad_out)
 {
-    krb5_error_code ret;
+    krb5_error_code ret = 0;
     krb5_authdata **list = NULL, **elements = NULL, **new_list;
     size_t i, n_elements, count = 0;