Fix misuse of PyUnicode_GET_SIZE, use PyUnicode_GET_LENGTH instead
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index a4536b1..83a2465 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -343,7 +343,7 @@
     if (line == NULL)
         return NULL;
 
-    if (PyUnicode_GET_SIZE(line) == 0) {
+    if (PyUnicode_GET_LENGTH(line) == 0) {
         /* Reached EOF */
         Py_DECREF(line);
         return NULL;