8213898: CDS dumping of springboot asserts in G1ArchiveAllocator::alloc_new_region

HeapRegionManager::find_highest_free needs to check if the region obtained from the HeapRegionManager::_regions is available.

Backport-of: 14b7467

Author: sunny868

src/hotspot/share/gc/g1/heapRegionManager.cpp

@@ -286,7 +286,7 @@ uint HeapRegionManager::find_highest_free(bool* expanded) {
   uint curr = max_length() - 1;
   while (true) {
     HeapRegion *hr = _regions.get_by_index(curr);
-    if (hr == NULL) {
+    if (hr == NULL || !is_available(curr)) {
       uint res = expand_at(curr, 1, NULL);
       if (res == 1) {
         *expanded = true;