[perl #111610] Trouble with XS-APItest/t/clone-with-stack.t

I ran into a bit of a problem when building perl-5.16.0.
'make test' showed a segfault in ext/XS-APItest/t/clone-with-stack.t.
It seems to be caused by accessing already freed memory, it
segfaults because I have MALLOC_PERTUBE_ set, thus glibc fills
freed memory with some value.

Digging deeper, it seems like perl_clone() does not fix
the cx's blk_oldcop element when doing context cloning, thus
blk_oldcop still points to PL_compiling in the old interp--the
calling scope for the BEGIN block being the compilation of the
code surrounding it--and the POPBLOCK done in leavesub will copy
the data from the old interp to PL_curcop.

After fixing this, it still crashed because interp_dup->Iop was
zero after the runops_standard() call (which is probably
correct as the end of the BEGIN block was reached). So I
also added an if statement that checks the pointer.

ext/XS-APItest/APItest.xs

@@ -3084,7 +3084,8 @@ CODE:
     PERL_SET_CONTEXT(interp_dup);
 
     /* continue after 'clone_with_stack' */
-    interp_dup->Iop = interp_dup->Iop->op_next;
+    if (interp_dup->Iop)
+	interp_dup->Iop = interp_dup->Iop->op_next;
 
     /* run with new perl */
     Perl_runops_standard(interp_dup);

sv.c

@@ -12312,6 +12312,7 @@ Perl_cx_dup(pTHX_ PERL_CONTEXT *cxs, I32 ix, I32 max, CLONE_PARAMS* param)
 	    Perl_croak(aTHX_ "Cloning substitution context is unimplemented");
 	}
 	else {
+	    ncx->blk_oldcop = (COP*)any_dup(ncx->blk_oldcop, param->proto_perl);
 	    switch (CxTYPE(ncx)) {
 	    case CXt_SUB:
 		ncx->blk_sub.cv		= (ncx->blk_sub.olddepth == 0