Fix undefined behavior of setjmp() macro.

[nanasi880]

Accepting the return value of a C setjmp macro as a variable is undefined behavior and should generally not be considered safe.

reference: https://pubs.opengroup.org/onlinepubs/009604599/functions/setjmp.html

An application shall ensure that an invocation of setjmp() appears in one of the following contexts only:
    - The entire controlling expression of a selection or iteration statement
    - One operand of a relational or equality operator with the other operand an integral constant expression, with the resulting expression being the entire controlling expression of a selection or iteration statement
    - The operand of a unary '!' operator with the resulting expression being the entire controlling expression of a selection or iteration
    - The entire expression of an expression statement (possibly cast to void)

If the invocation appears in any other context, the behavior is undefined.

It was introduced for debugging purposes in the following commit, and earlier implementations did not have this problem.
64bb59f

The implementation has been modified to conform to C standards while maintaining this debugging information where possible.
In the default case, we have given up on displaying current_stage.

[AkihiroSuda]

Thanks, but please sign the commit for DCO
https://github.com/apps/dco

(run git commit -a -s --amend, and make sure that the Signed-off-by: NAME <EMAIL> line with your real name is included in the commit message)

[nanasi880]

Completed sign the commit for DCO.

[kolyshkin]

Rebased for the sake of green CI

[nanasi880]

CI is all green :)

[cyphar]

lgtm