bpo-36328: Fix compiler warning in Py_NewInterpreter() (GH-12381)

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index c2d431c..49a2f18 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1434,7 +1434,7 @@
 PyThreadState *
 Py_NewInterpreter(void)
 {
-    PyThreadState *tstate;
+    PyThreadState *tstate = NULL;
     _PyInitError err = new_interpreter(&tstate);
     if (_Py_INIT_FAILED(err)) {
         _Py_ExitInitError(err);