Simplify principal access within libkrb5

For conciseness, directly use fields of krb5_principal objects instead
of using the accessor macros.
Avoid passing null pointers to memcpy/memcmp

By a strict reading of the C standard, memcpy and memcmp have
undefined behavior if their pointer arguments aren't valid object
pointers, even if the length argument is 0.  Compilers are becoming
more aggressive about breaking code with undefined behavior, so we
should try to avoid it when possible.

In a krb5_data object, we frequently use NULL as the data value when
the length is 0.  Accordingly, we should avoid copying from or
comparing the data field of a length-0 krb5_data object.  Add checks
to our wrapper functions (like data_eq and k5_memdup) and to code
which works with possibly-empty krb5_data objects.  In a few places,
use wrapper functions to simplify the code rather than adding checks.
Initialize outpos in krb5int_utf8_normalize

After k5memdup0 was slightly modified in
31124ffb81e8c0935403a9fdc169dead5ecaa777, some older versions of gcc
complain about outpos being possibly used before it is initialized.
This can't actually happen, but we can silence the error and also
simplify how outpos is initialized.