P4
In `G1RemSet::clean_card_before_refine`:
```
assert(!_g1h->is_gc_active(), "Only call concurrently");
...
assert(!SafepointSynchronize::is_at_safepoint(), "sanity");
```
```
assert(!_g1h->is_gc_active(), "Only call concurrently");
...
assert(!SafepointSynchronize::is_at_safepoint(), "sanity");
```
