clang: free error_message when finding the realm
The error message can come from either krb5_get_error_message(),
error_message() or a static string.
If krb5_get_error_message() is used then krb5_free_error_message()
needs to be called to free it.
We already strdup'd error_message() but there were some static
strings as well.
So unify around strdup'ing these strings so we can free() it
when the function exits.