Fix NULL pointer deref when creating iterator for non-iterable

src/types/iterator.rs

@@ -40,6 +40,10 @@ impl<'p> PyIterator<'p> {
     {
         unsafe {
             let ptr = ffi::PyObject_GetIter(obj.as_ptr());
+            // Returns NULL if an object cannot be iterated.
+            if ptr.is_null() {
+                return Err(PyDowncastError);
+            }
 
             if ffi::PyIter_Check(ptr) != 0 {
                 // this is not right, but this cause of segfault check #71

tests/test_various.rs

@@ -170,3 +170,13 @@ fn test_pickle() {
     "#
     );
 }
+
+#[test]
+fn incorrect_iter() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+    let int = 13isize.to_object(py);
+    let int_ref = int.as_ref(py);
+    // Should not segfault.
+    assert!(int_ref.iter().is_err());
+}