Improve dummy object filling condition in G1CollectedHeap::fill_archive_regions in cds

it's a minor fix/enhancement.

diff as below:

diff --git a/src/hotspot/share/gc/g1/g1CollectedHeap.cpp b/src/hotspot/share/gc/g1/g1CollectedHeap.cpp
index 5e0f4f8ac7d..299990cb052 100644
--- a/src/hotspot/share/gc/g1/g1CollectedHeap.cpp
+++ b/src/hotspot/share/gc/g1/g1CollectedHeap.cpp
@@ -705,7 +705,8 @@ void G1CollectedHeap::fill_archive_regions(MemRegion* ranges, size_t count) {
     // Fill the memory below the allocated range with dummy object(s),
     // if the region bottom does not match the range start, or if the previous
     // range ended within the same G1 region, and there is a gap.
- if (start_address != bottom_address) {
+ assert(start_address >= bottom_address, "bottom address should not be greater than start address");
+ if (start_address > bottom_address) {
       size_t fill_size = pointer_delta(start_address, bottom_address);
       G1CollectedHeap::fill_with_objects(bottom_address, fill_size);
       increase_used(fill_size * HeapWordSize);