xdump: Avoid undefined pointer arithmetic

xdump: Avoid undefined pointer arithmetic

s + n is defined only as long as it points into or just beyond the
same array as s.  Thus, our use of INT_MAX for "unlimited" relies on
undefined behavior.  Works fine in practice, but avoiding undefined
pointer arithmetic isn't hard here, so let's do it.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>