8236073: G1: Use SoftMaxHeapSize to guide GC heuristics

@@ -38,11 +38,11 @@
   HeapRegion* hr = _regions.get_by_address(addr);
   return hr;
 }
 
 inline HeapRegion* HeapRegionManager::at(uint index) const {
-  assert(is_available(index), "pre-condition");
+  assert(is_available(index) || is_in_concurrent_resizing(index), "pre-condition");
   HeapRegion* hr = _regions.get_by_index(index);
   assert(hr != NULL, "sanity");
   assert(hr->hrm_index() == index, "sanity");
   return hr;
 }