zustr2stp.h: Assert some assumptions about the size

zustr2stp.h: Assert some assumptions about the size

If the destination buffer is an array, we can check our assumptions.
This adds a readable way to explain that dsize must be strictly > ssize.
The reason is that the destination string is the source + '\0'.

If the destination is not an array, it's up to _FORTIFY_SOURCE or
-fanalyzer to catch newly introduced errors.  There's nothing we can do;
at least not portably.

Suggested-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>