patch 8.0.0227: crash with ff=dos when first line in file has no CR

Problem:    Crash when 'fileformat' is forced to "dos" and the first line in
            the file is empty and does not have a CR character.
Solution:   Don't check for CR before the start of the buffer.
diff --git a/src/testdir/test_fileformat.vim b/src/testdir/test_fileformat.vim
index de505d3..8dc25f6 100644
--- a/src/testdir/test_fileformat.vim
+++ b/src/testdir/test_fileformat.vim
@@ -17,7 +17,7 @@
 endfunc
 
 func Test_fileformat_autocommand()
-  let filecnt = ["\<CR>", "foobar\<CR>", "eins\<CR>", "\<CR>", "zwei\<CR>", "drei", "vier", "fünf", ""]
+  let filecnt = ["", "foobar\<CR>", "eins\<CR>", "\<CR>", "zwei\<CR>", "drei", "vier", "fünf", ""]
   let ffs = &ffs
   call writefile(filecnt, 'Xfile', 'b')
   au BufReadPre Xfile set ffs=dos ff=dos