patch 9.0.0882: using freed memory after SpellFileMissing autocmd uses bwipe

Problem:    Using freed memory after SpellFileMissing autocmd uses bwipe.
Solution:   Bail out if the window no longer exists.
diff --git a/src/spell.c b/src/spell.c
index 508c3ba..7d8ffe6 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -2110,8 +2110,8 @@
 	    {
 		spell_load_lang(lang);
 		// SpellFileMissing autocommands may do anything, including
-		// destroying the buffer we are using...
-		if (!bufref_valid(&bufref))
+		// destroying the buffer we are using or closing the window.
+		if (!bufref_valid(&bufref) || !win_valid_any_tab(wp))
 		{
 		    ret_msg = N_(e_spellfilemising_autocommand_deleted_buffer);
 		    goto theend;