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, 1 insertions, 1 deletions

diff --git a/weakmap.c b/weakmap.c
index 2145e4d2f8..1e86f22988 100644
--- a/weakmap.c
+++ b/weakmap.c
@@ -17,7 +17,7 @@ wmap_replace_ref(st_data_t *key, st_data_t *value, st_data_t _argp, int existing
 {
     *key = rb_gc_location((VALUE)*key);
 
-    VALUE *values = (VALUE *)value;
+    VALUE *values = (VALUE *)*value;
     VALUE size = values[0];
 
     for (VALUE index = 1; index <= size; index++) {