Fix in weak_count in Arc.

In the case the weak count was locked, the weak_count function could
return usize::MAX. We need to test this condition manually.

src/liballoc/arc.rs

@@ -453,7 +453,10 @@ impl<T: ?Sized> Arc<T> {
     #[inline]
     #[stable(feature = "arc_counts", since = "1.15.0")]
     pub fn weak_count(this: &Self) -> usize {
-        this.inner().weak.load(SeqCst) - 1
+        let cnt = this.inner().weak.load(SeqCst);
+        // If the weak count is currently locked, the value of the
+        // count was 0 just before taking the lock.
+        if cnt == usize::MAX { 0 } else { cnt - 1 }
     }
 
     /// Gets the number of strong (`Arc`) pointers to this value.