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

[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.