Fix GCC warning in Python/hamt.c (GH-7618)

diff --git a/Python/hamt.c b/Python/hamt.c
index f8bce59..562f777 100644
--- a/Python/hamt.c
+++ b/Python/hamt.c
@@ -2348,7 +2348,7 @@
         return NULL;
     }
 
-    PyHamtNode *new_root;
+    PyHamtNode *new_root = NULL;
 
     hamt_without_t res = hamt_node_without(
         (PyHamtNode *)(o->h_root),