G1 uses this closure to verify object references. It has the following issues:

- it uses decode_not_null which will assert if the given oop address is not in the heap, making the remainder of the verification useless in that case
- if the referenced object is not in the heap, we try to get its heap region too when printing, which also fails some assertions
- in the innermost if lots of code is duplicated in both cases

Fix these issues.