Fix mark overflow for regions (#53929)

Test GC\Scenarios\Boxing\variantlinklist\variantlinklist encountered a mark stack overflow that we didn't handle correctly in the regions case.

The problem was that this was a gen 2 GC, and we didn't iterate over the gen 0 and gen 1 regions, so the objects causing the mark stack overflow were in fact not revisited.

The fix is to iterate starting from gen 0 rather than the condemned generation.

I added an assert to make sure process_mark_overflow_internal has found at least one marked object to process - unfortunately, we can check this only for WKS GC.

src/coreclr/gc/gc.cpp

@@ -22290,7 +22290,7 @@ inline
 void gc_heap::mark_object (uint8_t* o THREAD_NUMBER_DCL)
 {
 #ifdef USE_REGIONS
-    if (is_in_condemned_gc (o))
+    if ((o != nullptr) && is_in_condemned_gc (o))
     {
         mark_object_simple (&o THREAD_NUMBER_ARG);
     }
@@ -23314,7 +23314,10 @@ void gc_heap::process_mark_overflow_internal (int condemned_gen_number,
 #endif //MULTIPLE_HEAPS
     BOOL  full_p = (condemned_gen_number == max_generation);
 
-        dprintf(3,("Processing Mark overflow [%Ix %Ix]", (size_t)min_add, (size_t)max_add));
+    dprintf(3,("Processing Mark overflow [%Ix %Ix]", (size_t)min_add, (size_t)max_add));
+
+    size_t obj_count = 0;
+
 #ifdef MULTIPLE_HEAPS
     for (int hi = 0; hi < n_heaps; hi++)
     {
@@ -23323,11 +23326,10 @@ void gc_heap::process_mark_overflow_internal (int condemned_gen_number,
 #else
     {
         gc_heap*  hp = 0;
-
 #endif //MULTIPLE_HEAPS
         int gen_limit = full_p ? total_generation_count : condemned_gen_number + 1;
 
-        for (int i = condemned_gen_number; i < gen_limit; i++)
+        for (int i = get_stop_generation_index (condemned_gen_number); i < gen_limit; i++)
         {
             generation* gen = hp->generation_of (i);
             heap_segment* seg = heap_segment_in_range (generation_start_segment (gen));
@@ -23347,6 +23349,7 @@ void gc_heap::process_mark_overflow_internal (int condemned_gen_number,
                     if (marked (o))
                     {
                         mark_through_object (o, TRUE THREAD_NUMBER_ARG);
+                        obj_count++;
                     }
 
                     o = o + Align (size (o), align_const);
@@ -23355,6 +23358,10 @@ void gc_heap::process_mark_overflow_internal (int condemned_gen_number,
                 seg = heap_segment_next_in_range (seg);
             }
         }
+#ifndef MULTIPLE_HEAPS
+        // we should have found at least one object
+        assert (obj_count > 0);
+#endif //MULTIPLE_HEAPS
     }
 }