[patch] Fix count_eh (with test)

[rg]

Currently, the count_eh opcode simply segfaults. After fixing the causes for the segfaults (included with the patch), I had to find out that the existing exception code now works differently. I therefor modelled a new Parrot_cx_count_handlers_local after the corresponding delete function.

I think the count_eh opcode would be useful for compiler tests, to see if any exception handlers were left on the stack without having to throw an exception.

[NotFound]

Applied in r36004