util.c: fix goto &somesub in $SIG{__DIE__} handlers

Custom die() handlers via $SIG{__DIE__} are supposed to be disabled during
their own execution, so throwing an exception from a $SIG{__DIE__}
handler does not recurse indefinitely.

This was implemented as a simple !CvDEPTH(cv) check, which basically
just verifies that the handler sub is not currently part of the call
stack. However, if the handler sub does not return normally, but uses
goto &othersub to transfer control to a different subroutine, this check
fails: CvDEPTH(cv) will be 0 (since the registered handler is not
running anymore), but the $SIG{__DIE__}->() call has not returned yet.

(Partial) fix: Locally (or rather temporarily) unset PL_diehook for the
duration of the handler call.

The reason this is not a full fix is that clearing PL_diehook is not
reflected in $SIG{__DIE__}, so any modification of $SIG{__DIE__}
including seeming no-ops such as { local $SIG{__DIE__}; } will reinstate
PL_diehook.

Fixes Perl#14527 (partially).

Author: mauke

util.c

@@ -1719,13 +1719,13 @@ Perl_invoke_exception_hook(pTHX_ SV *ex, bool warn)
     GV *gv;
     CV *cv;
     SV **const hook = warn ? &PL_warnhook : &PL_diehook;
-    /* sv_2cv might call Perl_croak() or Perl_warner() */
     SV * const oldhook = *hook;
 
     if (!oldhook || oldhook == PERL_WARNHOOK_FATAL)
         return FALSE;
 
     ENTER;
+    /* sv_2cv might call Perl_croak() or Perl_warner() */
     SAVESPTR(*hook);
     *hook = NULL;
     cv = sv_2cv(oldhook, &stash, &gv, 0);
@@ -1736,10 +1736,11 @@ Perl_invoke_exception_hook(pTHX_ SV *ex, bool warn)
 
         ENTER;
         save_re_context();
-        if (warn) {
-            SAVESPTR(*hook);
-            *hook = NULL;
-        }
+
+        /* call_sv(cv) might call Perl_croak() or Perl_warner() */
+        SAVESPTR(*hook);
+        *hook = NULL;
+
         exarg = newSVsv(ex);
         SvREADONLY_on(exarg);
         SAVEFREESV(exarg);