Make WeakMap 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_weakmap.rb

@@ -237,6 +237,10 @@ class TestWeakMap < Test::Unit::TestCase
     end;
   end
 
+  def test_gc_compact_stress
+    EnvUtil.under_gc_compact_stress { ObjectSpace::WeakMap.new }
+  end
+
   def test_replaced_values_bug_19531
     a = "A".dup
     b = "B".dup

weakmap.c

@@ -144,7 +144,9 @@ wmap_compact(void *ptr)
 {
     struct weakmap *w = ptr;
 
-    st_foreach(w->table, wmap_compact_table_i, (st_data_t)w->table);
+    if (w->table) {
+        st_foreach(w->table, wmap_compact_table_i, (st_data_t)w->table);
+    }
 }
 
 static const rb_data_type_t weakmap_type = {