spell: fix comparison of URL prefix with UTF-8 chars (issue #1739)

There is no problem in practice because the possible URL prefixes are hardcoded
in the spell plugin and don't contain any UTF-8 char on more than one byte.

src/plugins/spell/spell.c

@@ -379,8 +379,10 @@ spell_string_is_url (const char *word)
     for (i = 0; spell_url_prefix[i]; i++)
     {
         if (weechat_strncasecmp (word, spell_url_prefix[i],
-                                 strlen (spell_url_prefix[i])) == 0)
+                                 weechat_utf8_strlen (spell_url_prefix[i])) == 0)
+        {
             return 1;
+        }
     }
 
     /* word is not an URL */