patch 8.2.1521: reading past end of buffer when reading spellfile

Problem:    Reading past end of buffer when reading spellfile. (Yegappan
            Lakshmanan)
Solution:   Store the byte length and check for it.

src/spell.h

@@ -66,6 +66,7 @@ struct slang_S
     int		sl_add;		// TRUE if it's a .add file.
 
     char_u	*sl_fbyts;	// case-folded word bytes
+    long	sl_fbyts_len;	// length of sl_fbyts
     idx_T	*sl_fidxs;	// case-folded word indexes
     char_u	*sl_kbyts;	// keep-case word bytes
     idx_T	*sl_kidxs;	// keep-case word indexes