tree-wide: fix return value handling of base64mem()

tree-wide: fix return value handling of base64mem()

This returns an ssize_t, not an int. On populare archs that's the
difference between 64bit and 32bit. hence, let's be more careful here,
and not silently drop half the bits on the ground by assigning the
return value to "int".

As noticed by @malikabhi05:

https://github.com/systemd/systemd/pull/24754#discussion_r1062903159