Avoid using OP_NAME() in unimplemented_op(), as it won't give useful strings.

Avoid using OP_NAME() in unimplemented_op(), as it won't give useful strings.

Using OP_NAME() isn't going to be helpful here. Firstly, it doesn't cope
with out of range op numbers - it only "special" cases op_custom.  Secondly,
as the three ops we "panic" on are padmy, mapstart and custom, if we get
here for a custom op then that means that the custom op didn't have an
implementation. Given that OP_NAME() looks up the custom op by its pp_addr,
likely it will return NULL, unless someone (unhelpfully) registers
&PL_unimplemented_op as the address of their custom op.  NULL doesn't
generate a useful error message. "custom" does. */