6812428: G1: Error: assert(ret || obj_in_cs(obj),"sanity")

Summary: The length of the fast cset test vector is decided at the beginning of a GC, but more regions can be added during the GC. The simple fix is to set the length of the fast cset test vector to the max.
Reviewed-by: iveresov

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -2509,7 +2509,7 @@ G1CollectedHeap::do_collection_pause_at_safepoint(HeapRegion* popular_region) {
 
     guarantee(_in_cset_fast_test == NULL, "invariant");
     guarantee(_in_cset_fast_test_base == NULL, "invariant");
-    _in_cset_fast_test_length = n_regions();
+    _in_cset_fast_test_length = max_regions();
     _in_cset_fast_test_base =
                              NEW_C_HEAP_ARRAY(bool, _in_cset_fast_test_length);
     memset(_in_cset_fast_test_base, false,