Your code has undefined behavior because you cannot obtain your j_ptr
by doing i_ptr + 1. A past the end pointer is not a pointer to the
next member.

However, the example is correct without j_ptr. It is possible to
obtain a pointer to the first member of a standard-layout class such
as MyClass, even if that member is private.

I wouldn't really call it a security hole. The only way you will do
this in idiomatic C++ code is with reinterpret_cast, and
reinterpret_cast is a giant red flag which says "I'm an expert, I know
what I'm doing, and what I'm doing is potentially dangerous". This
never happens by accident.