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.

src/fileio.c

@@ -2251,8 +2251,9 @@ rewind_retry:
 			len = (colnr_T)(ptr - line_start + 1);
 			if (fileformat == EOL_DOS)
 			{
-			    if (ptr[-1] == CAR)	/* remove CR */
+			    if (ptr > line_start && ptr[-1] == CAR)
 			    {
+				/* remove CR before NL */
 				ptr[-1] = NUL;
 				--len;
 			    }

src/testdir/test_fileformat.vim

@@ -17,7 +17,7 @@ func Test_fileformat_after_bw()
 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

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    227,
 /**/
     226,
 /**/