py, extmod: Add explicit initializers for default values.

When compiling with -Wextra which includes -Wmissing-field-initializers
GCC will warn that the defval field of mp_arg_val_t is not initialized.
This is just a warning as it is defined to be zero initialized, but since
it is a union it makes sense to be explicit about which member we're
going to use, so add the explicit initializers and get rid of the
warning.

extmod/machine_i2c.c

@@ -311,8 +311,8 @@ STATIC void mp_machine_soft_i2c_print(const mp_print_t *print, mp_obj_t self_in,
 STATIC void machine_i2c_obj_init_helper(machine_i2c_obj_t *self, size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
     enum { ARG_scl, ARG_sda, ARG_freq, ARG_timeout };
     static const mp_arg_t allowed_args[] = {
-        { MP_QSTR_scl, MP_ARG_REQUIRED | MP_ARG_OBJ },
-        { MP_QSTR_sda, MP_ARG_REQUIRED | MP_ARG_OBJ },
+        { MP_QSTR_scl, MP_ARG_REQUIRED | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL} },
+        { MP_QSTR_sda, MP_ARG_REQUIRED | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL} },
         { MP_QSTR_freq, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 400000} },
         { MP_QSTR_timeout, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 255} },
     };

extmod/vfs_lfs.c

@@ -35,7 +35,7 @@
 enum { LFS_MAKE_ARG_bdev, LFS_MAKE_ARG_readsize, LFS_MAKE_ARG_progsize, LFS_MAKE_ARG_lookahead, LFS_MAKE_ARG_mtime };
 
 static const mp_arg_t lfs_make_allowed_args[] = {
-    { MP_QSTR_, MP_ARG_REQUIRED | MP_ARG_OBJ },
+    { MP_QSTR_, MP_ARG_REQUIRED | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL} },
     { MP_QSTR_readsize, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 32} },
     { MP_QSTR_progsize, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 32} },
     { MP_QSTR_lookahead, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 32} },

py/modmath.c

@@ -206,8 +206,8 @@ MATH_FUN_1(lgamma, lgamma)
 STATIC mp_obj_t mp_math_isclose(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
     enum { ARG_a, ARG_b, ARG_rel_tol, ARG_abs_tol };
     static const mp_arg_t allowed_args[] = {
-        {MP_QSTR_, MP_ARG_REQUIRED | MP_ARG_OBJ},
-        {MP_QSTR_, MP_ARG_REQUIRED | MP_ARG_OBJ},
+        {MP_QSTR_, MP_ARG_REQUIRED | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL}},
+        {MP_QSTR_, MP_ARG_REQUIRED | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL}},
         {MP_QSTR_rel_tol, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = MP_OBJ_NULL}},
         {MP_QSTR_abs_tol, MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = MP_OBJ_NEW_SMALL_INT(0)}},
     };