extmod/machine_signal: Fix fault when no args are passed to Signal().

extmod/machine_signal.c

@@ -43,13 +43,13 @@ typedef struct _machine_signal_t {
 } machine_signal_t;
 
 STATIC mp_obj_t signal_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
-    mp_obj_t pin = args[0];
+    mp_obj_t pin;
     bool invert = false;
 
     #if defined(MICROPY_PY_MACHINE_PIN_MAKE_NEW)
     mp_pin_p_t *pin_p = NULL;
 
-    if (mp_obj_is_obj(pin)) {
+    if (n_args > 0 && mp_obj_is_obj(args[0])) {
         mp_obj_base_t *pin_base = (mp_obj_base_t*)MP_OBJ_TO_PTR(args[0]);
         pin_p = (mp_pin_p_t*)pin_base->type->protocol;
     }
@@ -96,6 +96,7 @@ STATIC mp_obj_t signal_make_new(const mp_obj_type_t *type, size_t n_args, size_t
     // Otherwise there should be 1 or 2 args
     {
         if (n_args == 1) {
+            pin = args[0];
             if (n_kw == 0) {
             } else if (n_kw == 1 && args[1] == MP_OBJ_NEW_QSTR(MP_QSTR_invert)) {
                 invert = mp_obj_is_true(args[2]);