Fix a valgrind regression in chunk_recycle()

Fix a latent valgrind bug exposed by d412624b25eed2b5c52b7d94a71070d3aab03cb4
(Move retaining out of default chunk hooks).
diff --git a/src/chunk.c b/src/chunk.c
index f292c98..dff537f 100644
--- a/src/chunk.c
+++ b/src/chunk.c
@@ -316,10 +316,11 @@
 			size_t i;
 			size_t *p = (size_t *)(uintptr_t)ret;
 
-			JEMALLOC_VALGRIND_MAKE_MEM_DEFINED(ret, size);
 			for (i = 0; i < size / sizeof(size_t); i++)
 				assert(p[i] == 0);
 		}
+		if (config_valgrind)
+			JEMALLOC_VALGRIND_MAKE_MEM_DEFINED(ret, size);
 	}
 	return (ret);
 }