Fix zero free objects assertion

This commit is to attempt fixing this error:

  http://ci.rvm.jp/results/trunk-gc-asserts@ruby-sky1/2353281

Each non-full heap_page struct contains a reference to the next page
that contains free slots.  Compaction could fill any page, including
pages that happen to be linked to as "pages which contain free slots".

To fix this, we'll iterate each page, and rebuild the "free page list"
depending on the number of actual free slots on that page.  If there are
no free slots on the page, we'll set the free_next pointer to NULL.

Finally we'll pop one page off the "free page list" and set it as the
"using page" for the next allocation.

gc.c

@@ -8553,14 +8553,14 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl
         heap_add_pages(objspace, heap_eden, heap_allocated_pages);
     }
 
-    VALUE moved_list;
+    VALUE moved_list_head;
     VALUE disabled = rb_gc_disable();
 
     if (use_toward_empty) {
-        moved_list = gc_compact_heap(objspace, compare_free_slots);
+        moved_list_head = gc_compact_heap(objspace, compare_free_slots);
     }
     else {
-        moved_list = gc_compact_heap(objspace, compare_pinned);
+        moved_list_head = gc_compact_heap(objspace, compare_pinned);
     }
     heap_eden->freelist = NULL;
 
@@ -8576,27 +8576,44 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl
     heap_eden->free_pages = NULL;
     heap_eden->using_page = NULL;
 
-    while (moved_list) {
+    /* For each moved slot */
+    while (moved_list_head) {
         VALUE next_moved;
         struct heap_page *page;
 
-        page = GET_HEAP_PAGE(moved_list);
-        next_moved = RMOVED(moved_list)->next;
+        page = GET_HEAP_PAGE(moved_list_head);
+        next_moved = RMOVED(moved_list_head)->next;
 
-        RMOVED(moved_list)->flags = 0;
-        RMOVED(moved_list)->destination = 0;
-        RMOVED(moved_list)->next = 0;
+        /* clear the memory for that moved slot */
+        RMOVED(moved_list_head)->flags = 0;
+        RMOVED(moved_list_head)->destination = 0;
+        RMOVED(moved_list_head)->next = 0;
         page->free_slots++;
-        heap_page_add_freeobj(objspace, page, moved_list);
+        heap_page_add_freeobj(objspace, page, moved_list_head);
+
         if (page->free_slots == page->total_slots && heap_pages_freeable_pages > 0) {
             heap_pages_freeable_pages--;
 	    heap_unlink_page(objspace, heap_eden, page);
 	    heap_add_page(objspace, heap_tomb, page);
-        } else if (page->free_slots == page->total_slots) {
-            page->free_next = NULL;
         }
         objspace->profile.total_freed_objects++;
-        moved_list = next_moved;
+        moved_list_head = next_moved;
+    }
+
+    /* Add any eden pages with free slots back to the free pages list */
+    struct heap_page *page = NULL;
+    list_for_each(&heap_eden->pages, page, page_node) {
+        if (page->free_slots > 0) {
+            heap_add_freepage(objspace, heap_eden, page);
+        } else {
+            page->free_next = NULL;
+        }
+    }
+
+    /* Set up "using_page" if we have any pages with free slots */
+    if (heap_eden->free_pages) {
+        heap_eden->using_page = heap_eden->free_pages;
+        heap_eden->free_pages = heap_eden->free_pages->free_next;
     }
 
     if (use_verifier) {