strxcpyx: don't overflow dest on strpcpyf truncate

strxcpyx: don't overflow dest on strpcpyf truncate

When vsnprintf() truncated output, dest was advanced by the entire
size of dest leaving it just past the end.  Then the fall-through \0
termination scribbled one past the end.  The explicit null termination
is not necessary since vsnprintf() always includes the terminator even
when truncated.

Additionally these functions encourage calling with zero-length sizes,
while assuming non-zero sizes with potential buffer overflows.

Simply short-circuit the relevant functions when size == 0.

Fixes https://github.com/systemd/systemd/issues/6252