correct initialization code (#3376)

Explicitly initialize struct members rather than relying on compiler extensions.
diff --git a/Python/pystate.c b/Python/pystate.c
index 3d32077..2d92637 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -37,8 +37,7 @@
 void
 _PyRuntimeState_Init(_PyRuntimeState *runtime)
 {
-    _PyRuntimeState initial = {};
-    *runtime = initial;
+    memset(runtime, 0, sizeof(*runtime));
 
     _PyObject_Initialize(&runtime->obj);
     _PyMem_Initialize(&runtime->mem);