bpo-39500: Fix compile warnings in unicodeobject.c (GH-18519)

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 11fa1fb5..4475eca 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -12207,8 +12207,8 @@
         return 0;
     }
 
-    int kind;
-    void *data;
+    int kind = 0;
+    void *data = NULL;
     wchar_t *wstr;
     if (ready) {
         kind = PyUnicode_KIND(self);