fixes: vim/vim#17474closes: vim/vim#1768332a1b26ef3
vim-patch:41ee98c: runtime(filetype): fix incorrect pattern and break early
- Using `\n` is incorrect, as result of getline() does not contain line
breaks and only uses `\n` for NUL bytes.
- Return when b:asmsyntax is set, like many other filetypes.
closes: vim/vim#1770641ee98c3c5
Co-authored-by: Wu Yongwei <wuyongwei@gmail.com>