Rust: add Drop impl for p_context_t

spec/test_tree_delete_token_node_memory.rs

@@ -10,8 +10,18 @@ fn main() {
     assert!(start.b().valid());
     assert_eq!(2, unsafe { *start.b().pvalue() });
 
-    /* The free_token_node code block runs from p_context_delete(), not before. */
+    /* The free_token_node code block runs when the context is disposed of, not
+     * before, and frees each of the two token nodes exactly once. */
     assert_eq!(0, FREED.load(Ordering::SeqCst));
     p_context_delete(context);
     assert_eq!(2, FREED.load(Ordering::SeqCst));
+
+    /* Letting the context go out of scope runs the code block too, so a caller
+     * which never calls p_context_delete() does not leak. */
+    {
+        let mut context = p_context_new(b"ab");
+        assert_eq!(P_SUCCESS, p_parse(&mut context));
+        assert_eq!(2, FREED.load(Ordering::SeqCst));
+    }
+    assert_eq!(4, FREED.load(Ordering::SeqCst));
 }