ada: Add an assert in Posix Interrupt_Wait

All functions but Interrupt_Wait in s-inmaop__posix are checking the
result of their syscalls with an assert. However, any return code of
sigwait different than 0 means that something went wrong for it.

From sigwait man:
> RETURN VALUE
>      On success, sigwait() returns 0.  On  error,  it  returns  a
>      positive error number (listed in ERRORS).

gcc/ada/

	* libgnarl/s-inmaop__posix.adb: Add assert after sigwait in
	Interrupt_Wait

gcc/ada/libgnarl/s-inmaop__posix.adb

@@ -135,6 +135,7 @@ package body System.Interrupt_Management.Operations is
 
    begin
       Result := sigwait (Mask, Sig'Access);
+      pragma Assert (Result = 0);
 
       if Result /= 0 then
          return 0;