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;
 			    }