Fix stack collapse on new object BUILD

[Etsukata]

Current implementation of calling BUILD from XS discards scalar by its own POPs. However, it can unexpectedly decrement PL_stack_sp and lead to stack collapse, especially 'STACK UNDERFLOW!!!'. Fix discards on BUILD and also DEMOLISH using G_DISCARD flag according to the perldoc documentation.

• http://perldoc.perl.org/perlcall.html

Stack Snapshot

• -Ds output of perl-5.24.1 t/900_mouse_bugs/020_stack_collapse_on_build.t
• with this fix

  ... snip ....
  =>  *  IV(2)  PV("My::ChildB")  *  \HV()  PV("strict_constructor")  
  =>  *  IV(2)  PV("My::ChildB")  *  \HV()  CV(strict_constructor)  
  =>  *  IV(2)  PV("My::ChildB")  \HV()  \HV()  
  =>  *  IV(2)  PV("My::ChildB")  
  =>  *  IV(2)  PV("My::ChildB")  \HV()  \HV()  
  =>  *  IV(2)  PV("My::ChildB")  
  =>  *  IV(2)  PV("My::ChildB")  \HV()  \HV()  
  =>  *  IV(2)  PV("My::ChildB")  
  =>  *  IV(2)  \HV()  
  =>  *  IV(2)  
  =>  *  IV(2)  *  
  =>  *  IV(2)  *  IV(1)  
  =>  *  IV(2)  IV(1)  
  =>  *  IV(2)  
  =>  *  IV(2)  
  =>  *  IV(2)  
  =>  *  IV(2)  IV(2)  
  =>  *  IV(2)  IV(2)  GV()  
  =>  IV(2)  IV(2)  
  =>  
  ... snip ...

• without this fix

  =>  *  IV(2)  PV("My::ChildB")  *  \HV()  PV("strict_constructor")  
  =>  *  IV(2)  PV("My::ChildB")  *  \HV()  CV(strict_constructor)  
  =>  *  IV(2)  PV("My::ChildB")  \HV()  \HV()  
  =>  *  IV(2)  PV("My::ChildB")  
  =>  *  IV(2)  \HV()  \HV()  
  =>  *  IV(2)  
  =>  *  \HV()  \HV()  
  =>  *  
  =>  *  \HV()  \HV()  
  =>  *  (FREED)  
  =>  *  (FREED)  *  
  =>  *  (FREED)  *  IV(1)  
  =>  *  (FREED)  IV(1)  
  Use of uninitialized value in multiplication (*) at /path/to/p5-Mouse/t/900_mouse_bugs/020_stack_collapse_on_build.t line 31.
  =>  *  NV(0)  
  =>  *  NV(0)  
  =>  *  NV(0)  
  =>  *  NV(0)  IV(2)  
  =>  *  NV(0)  IV(2)  GV()  
  =>  NV(0)  IV(2)  
  =>  

[syohex]

Looks good. Thanks.

[syohex]

I have released new version.

https://metacpan.org/release/SYOHEX/Mouse-v2.4.8

[Etsukata]

Great! Thanks.