Fix chunk_recycle() Valgrind integration.

Fix chunk_recycyle() to unconditionally inform Valgrind that returned
memory is undefined.  This fixes Valgrind warnings that would result
from a huge allocation being freed, then recycled for use as an arena
chunk.  The arena code would write metadata to the chunk header, and
Valgrind would consider these invalid writes.
diff --git a/ChangeLog b/ChangeLog
index 3dff01e..374459d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -10,6 +10,8 @@
 
   Bug fixes:
   - Fix "arenas.extend" mallctl to output the number of arenas.
+  - Fix chunk_recycyle() to unconditionally inform Valgrind that returned memory
+    is undefined.
 
 * 3.2.0 (November 9, 2012)