Fix crash during compaction

[Bug #19529]

The fix for [Bug #19529] in commit 548086b contained a bug that crashes
on the following script:

```
wm = ObjectSpace::WeakMap.new
obj = Object.new
100.times do
  wm[Object.new] = obj
  GC.start
end
GC.compact
```

1 files changed, 10 insertions, 0 deletions

diff --git a/test/ruby/test_weakmap.rb b/test/ruby/test_weakmap.rb
index d84eb8241e..9bbe2d6b81 100644
--- a/test/ruby/test_weakmap.rb
+++ b/test/ruby/test_weakmap.rb
@@ -183,5 +183,15 @@ class TestWeakMap < Test::Unit::TestCase
       GC.compact
       @wm[i] = obj
     end
+
+    assert_separately(%w(--disable-gems), <<-'end;')
+      wm = ObjectSpace::WeakMap.new
+      obj = Object.new
+      100.times do
+        wm[Object.new] = obj
+        GC.start
+      end
+      GC.compact
+    end;
   end
 end