py/modthread: Initialize thread state nlr_top to NULL.

This ensures the check in MP_NLR_JUMP_HEAD works as expected and
nlr_jump_fail gets called so we get a bit better error message.

Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>

py/runtime.h

@@ -165,6 +165,7 @@ static inline void mp_thread_init_state(mp_state_thread_t *ts, size_t stack_size
     ts->gc_lock_depth = 0;
 
     // There are no pending jump callbacks or exceptions yet
+    ts->nlr_top = NULL;
     ts->nlr_jump_callback_top = NULL;
     ts->mp_pending_exception = MP_OBJ_NULL;