Fix off-by-one error

Author: mpage

Lib/test/test_frame.py

@@ -602,11 +602,13 @@ def test_overwrite_locals(self):
         # from an ancestor in the call stack.
         def f():
             xs = [1, 2, 3]
+            ys = [4, 5, 6]
             return g(xs)
 
         def g(xs):
             f = sys._getframe()
             f.f_back.f_locals["xs"] = None
+            f.f_back.f_locals["ys"] = None
             return xs[1]
 
         self.assertEqual(f(), 2)

Objects/frameobject.c

@@ -209,12 +209,16 @@ framelocalsproxy_getitem(PyObject *self, PyObject *key)
 static int
 add_overwritten_fast_local(PyFrameObject *frame, PyObject *obj)
 {
-    Py_ssize_t new_size = 1;
-    if (frame->f_overwritten_fast_locals != NULL) {
-        new_size = PyTuple_Size(frame->f_overwritten_fast_locals);
-        if (new_size == -1) {
+    Py_ssize_t new_size;
+    if (frame->f_overwritten_fast_locals == NULL) {
+        new_size = 1;
+    }
+    else {
+        Py_ssize_t size = PyTuple_Size(frame->f_overwritten_fast_locals);
+        if (size == -1) {
             return -1;
         }
+        new_size = size + 1;
     }
     PyObject *new_tuple = PyTuple_New(new_size);
     if (new_tuple == NULL) {