sd-bus: make strict asan shut up

sd-bus: make strict asan shut up

asan doesn't like it if we use strndup() (i.e. a string function) on a
non-NULL terminated buffer (i.e. something that isn't really a string).

Let's hence use memdup_suffix0() instead of strndup(), which is more
appropriate for binary data that is to become a string.

Fixes: #10385