xgetXXbyYY: Simplify elifs

xgetXXbyYY: Simplify elifs

-  Use SIZE_MAX rather than (size_t)-1, to improve readability.

-  Move the only branch that breaks to the first place, so that we
   remove an else.  This reduces nesting while parsing the code.

-  Now that we only have a 2-branch conditional where both branches
   assign to the same variable, rewrite it as a ternary, to shorten.

Signed-off-by: Alejandro Colomar <alx@kernel.org>