S_process_special_blocks() should use a new stack for BEGIN blocks.

This avoids the stack moving underneath anything that directly or indirectly
calls Perl_load_module().

[Committer’s note:

 This fixes bug #119993.

 Furthermore, under STRESS_REALLOC, t/io/layers.t was crashing like this:

 $ ./perl -Ilib -e ' open(UTF, "<:raw:encoding(utf8)", 'tmp75851B') or die $!; ref #blahblahblahblahblahblahblahblahblah'
 Segmentation fault: 11

 (The comment seems to be necessary to make it crash.)

 It was happening because open() was causing a module to be loaded
 while the arguments to open() were still on the stack.
]

op.c

@@ -7957,8 +7957,10 @@ S_process_special_blocks(pTHX_ I32 floor, const char *const fullname,
     if (*name == 'B') {
 	if (strEQ(name, "BEGIN")) {
 	    const I32 oldscope = PL_scopestack_ix;
+            dSP;
 	    if (floor) LEAVE_SCOPE(floor);
 	    ENTER;
+            PUSHSTACKi(PERLSI_REQUIRE);
 	    SAVECOPFILE(&PL_compiling);
 	    SAVECOPLINE(&PL_compiling);
 	    SAVEVPTR(PL_curcop);
@@ -7968,6 +7970,7 @@ S_process_special_blocks(pTHX_ I32 floor, const char *const fullname,
 	    GvCV_set(gv,0);		/* cv has been hijacked */
 	    call_list(oldscope, PL_beginav);
 
+            POPSTACK;
 	    LEAVE;
 	}
 	else