fix: use no_ff instead of ffdos as condition

Problem: line2byte behavior is changed after commit b051b13. It no
longer return `-1` on empty buffer.

Solution: use `nof_ff` instead of `!ff_dos` as condition. Then
compatible behavior of line2byte() is restored.

test/functional/api/buffer_spec.lua

@@ -1876,6 +1876,7 @@ describe('api/buf', function()
     it('works in empty buffer', function()
       eq(0, get_offset(0))
       eq(1, get_offset(1))
+      eq(-1, funcs.line2byte('$'))
     end)
 
     it('works in buffer with one line inserted', function()