ConstantPool::print_on can crash if _cache is NULL

ConstantPool::print_on does:

 st->print_cr(" - resolved_references: " INTPTR_FORMAT, p2i(resolved_references()));

but resolved_references() does:

return (objArrayOop)_cache->resolved_references();

which will crash is _cache is NULL.

print_on should be using resolved_references_or_null().

It would also be useful for print_on to report the CP length before starting to iterate the contents.