Fix bas_program.c build warning under macos

bas_program.c:234:35: error: adding 'size_t' (aka 'unsigned long') to a string does not append to the string [-Werror,-Wstring-plus-int]
      FS_putChars(chn, "        " + len);
                       ~~~~~~~~~~~^~~~~
bas_program.c:234:35: note: use array indexing to silence this warning
      FS_putChars(chn, "        " + len);
                                  ^
                       &          [    ]
1 error generated.

Signed-off-by: liuhaitao <liuhaitao@xiaomi.com>
1 file changed