prepare_export_lexical: save PL_comppad safely

When the pad being saved and the pad for PL_compcv is the same,
in some cases the actual exports would result in reallocating the
AvARRAY() for the saved PL_comppad.

The LEAVE in finish_export_lexical() would restore the old PL_comppad
(which is fine) and the pre-reallocation PL_curpad (which isn't fine).

This would later panic.

SAVECOMPPAD; restores PL_comppad on LEAVE and then restores PL_curpad
from PL_comppad, preventing the desync between those values.

It's unclear to me why only the save_BEGINs; causes this, but the fix
does fix a real problem and prevents the panics that I'm aware of
here.

Fixes #21981

builtin.c

@@ -48,8 +48,9 @@ Perl_prepare_export_lexical(pTHX)
     /* We need to have PL_comppad / PL_curpad set correctly for lexical importing */
     ENTER;
     SAVESPTR(PL_comppad_name); PL_comppad_name = PadlistNAMES(CvPADLIST(PL_compcv));
-    SAVESPTR(PL_comppad);      PL_comppad      = PadlistARRAY(CvPADLIST(PL_compcv))[1];
-    SAVESPTR(PL_curpad);       PL_curpad       = PadARRAY(PL_comppad);
+    SAVECOMPPAD();
+    PL_comppad      = PadlistARRAY(CvPADLIST(PL_compcv))[1];
+    PL_curpad       = PadARRAY(PL_comppad);
 }
 
 #define export_lexical(name, sv)  S_export_lexical(aTHX_ name, sv)

lib/builtin.t

@@ -621,6 +621,15 @@ TODO: {
     }
 }
 
+# github #21981
+{
+    fresh_perl_is(<<'EOS', "", {}, "github 21981: panic in intro_my");
+use B;
+BEGIN { B::save_BEGINs; }
+use v5.39;
+EOS
+}
+
 # vim: tabstop=4 shiftwidth=4 expandtab autoindent softtabstop=4
 
 done_testing();