Fix a bug in never_loop when anonymous blocks are nested in named blocks

The following code

```
loop {
    'a: {
        { }
        break 'a;
    }
}
```

was detected as a never-looping loop.

clippy_lints/src/loops/never_loop.rs

@@ -163,7 +163,9 @@ fn never_loop_expr(expr: &Expr<'_>, ignore_ids: &mut Vec<HirId>, main_loop_id: H
                 ignore_ids.push(b.hir_id);
             }
             let ret = never_loop_block(b, ignore_ids, main_loop_id);
-            ignore_ids.pop();
+            if l.is_some() {
+                ignore_ids.pop();
+            }
             ret
         },
         ExprKind::Continue(d) => {

tests/ui/never_loop.rs

@@ -250,6 +250,15 @@ pub fn test20() {
     }
 }
 
+pub fn test21() {
+    loop {
+        'a: {
+            { }
+            break 'a;
+        }
+    }
+}
+
 fn main() {
     test1();
     test2();