Fix test code in jamallocator

With the update of rand to 0.7, the test code of jemallocator needed to
be fixed, but the fix was missing in the commit.  This commit adds it.

Author: statiolake

tests/test_jemallocator.rs

@@ -25,7 +25,7 @@ fn do_allocate_heap() -> UnitResult {
 
     let mut rng = SmallRng::from_rng(thread_rng())?;
 
-    let v = rng
+    let v = (&mut rng)
         .sample_iter(&rand::distributions::Standard)
         .take(SIZE)
         .collect::<Vec<usize>>();