Perl_custom_op_get_field(): remove undef behaviour

Perl_custom_op_get_field(): remove undef behaviour

Thus function has a couple a switches with

               default:
                   NOT_REACHED; /* NOTREACHED */

but clang is complaining that the value returned by the function is
undefined if those default branches are taken, since the 'any' variable
doesn't get set in that path.

Replace the NOTREACHED with a croak("panic: ..."). It's possible (albeit
not intended) for Perl_custom_op_get_field() to be called with a 'field'
arg which triggers the default case. So if this ever happens, make it
clear that something has gone wrong, rather than just silently
continuing on non-debugging builds.

In any case, this shuts up clang.