c.h: really avoid undefined behavior in SINT_MAX

c.h: really avoid undefined behavior in SINT_MAX

The previous fix in #2013 still invoked undefined behavior by shifting
into the sign-bit.

Now we have a correct, albeit unwieldy solution that avoids undefined
behavior.
Instead of bit fiddling it uses plain addition and substraction.

We are looking for (in LaTeX notation):
  2^(n - 1) - 1
= 2 * 2^(n - 2) - 1
= 2^(n - 2) + 2^(n - 2) - 1
= 2^(n - 2) - 1 + 2^(n - 2)