Make WeakKeyMap safe for compaction during allocation

During allocation, the table may not have been allocated yet which would
crash in the st_foreach.

test/ruby/test_weakkeymap.rb

@@ -123,6 +123,10 @@ class TestWeakKeyMap < Test::Unit::TestCase
     end;
   end
 
+  def test_gc_compact_stress
+    EnvUtil.under_gc_compact_stress { ObjectSpace::WeakKeyMap.new }
+  end
+
   private
 
   def assert_weak_include(m, k, n = 100)

weakmap.c

@@ -609,7 +609,9 @@ wkmap_compact(void *ptr)
 {
     struct weakkeymap *w = ptr;
 
+    if (w->table) {
         st_foreach_with_replace(w->table, wkmap_compact_table_i, wkmap_compact_table_replace, (st_data_t)0);
+    }
 }
 
 static const rb_data_type_t weakkeymap_type = {