Compacting the heap can cause GC, so disable it

When we compact the heap, various st tables are updated, particularly
the table that contains the object id map.  Updating an st table can
cause a GC to occur, and we need to prevent any GC from happening while
moving or updating references.

gc.c

@@ -8554,6 +8554,7 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl
     }
 
     VALUE moved_list;
+    VALUE disabled = rb_gc_disable();
 
     if (use_toward_empty) {
         moved_list = gc_compact_heap(objspace, compare_free_slots);
@@ -8563,7 +8564,6 @@ gc_compact_after_gc(rb_objspace_t *objspace, int use_toward_empty, int use_doubl
     }
     heap_eden->freelist = NULL;
 
-    VALUE disabled = rb_gc_disable();
     gc_update_references(objspace);
     if (!RTEST(disabled)) rb_gc_enable();