[core] Fix segfault in FixedPMCArray's set_pmc vtable method. It would assume that something that does array was either a FixedPMCArray or something with the same structure; a bad assumption if you subclassed it with a high level object.
[core] Fix segfault in FixedPMCArray's set_pmc vtable method. It would assume that something that does array was either a FixedPMCArray or something with the same structure; a bad assumption if you subclassed it with a high level object.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@26932 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot