patch 8.2.4919: can add invalid bytes with :spellgood

Problem:    Can add invalid bytes with :spellgood.
Solution:   Check for a valid word string.

src/errors.h

@@ -3273,3 +3273,7 @@ EXTERN char e_stray_closing_curly_str[]
 EXTERN char e_missing_close_curly_str[]
 	INIT(= N_("E1279: Missing '}': %s"));
 #endif
+#ifdef FEAT_SPELL
+EXTERN char e_illegal_character_in_word[]
+	INIT(= N_("E1280: Illegal character in word"));
+#endif

src/mbyte.c

@@ -4226,7 +4226,7 @@ utf_find_illegal(void)
     convert_setup(&vimconv, NULL, NULL);
 }
 
-#if defined(FEAT_GUI_GTK) || defined(PROTO)
+#if defined(FEAT_GUI_GTK) || defined(FEAT_SPELL) || defined(PROTO)
 /*
  * Return TRUE if string "s" is a valid utf-8 string.
  * When "end" is NULL stop at the first NUL.

src/spellfile.c

@@ -4390,6 +4390,10 @@ store_word(
     int		res = OK;
     char_u	*p;
 
+    // Avoid adding illegal bytes to the word tree.
+    if (enc_utf8 && !utf_valid_string(word, NULL))
+	return FAIL;
+
     (void)spell_casefold(curwin, word, len, foldword, MAXWLEN);
     for (p = pfxlist; res == OK; ++p)
     {
@@ -6190,6 +6194,12 @@ spell_add_word(
     int		i;
     char_u	*spf;
 
+    if (enc_utf8 && !utf_valid_string(word, NULL))
+    {
+	emsg(_(e_illegal_character_in_word));
+	return;
+    }
+
     if (idx == 0)	    // use internal wordlist
     {
 	if (int_wordlist == NULL)

src/testdir/test_spell_utf8.vim

@@ -780,5 +780,10 @@ func Test_no_crash_with_weird_text()
   bwipe!
 endfunc
 
+" Invalid bytes may cause trouble when creating the word list.
+func Test_check_for_valid_word()
+  call assert_fails("spellgood! 0\xac", 'E1280:')
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4919,
 /**/
     4918,
 /**/