vim-patch:8.2.0097: crash with autocommand and spellfile

Problem:    Crash with autocommand and spellfile. (Tim Pope)
Solution:   Do not pop exestack when not pushed. (closes vim/vim#5450)
ce6db0273f

src/nvim/spellfile.c

@@ -580,6 +580,7 @@ slang_T *spell_load_file(char_u *fname, char_u *lang, slang_T *old_lp, bool sile
   slang_T *lp = NULL;
   int c = 0;
   int res;
+  bool did_estack_push = false;
 
   fd = os_fopen((char *)fname, "r");
   if (fd == NULL) {
@@ -612,6 +613,7 @@ slang_T *spell_load_file(char_u *fname, char_u *lang, slang_T *old_lp, bool sile
 
   // Set sourcing_name, so that error messages mention the file name.
   estack_push(ETYPE_SPELL, (char *)fname, 0);
+  did_estack_push = true;
 
   // <HEADER>: <fileID>
   const int scms_ret = spell_check_magic_string(fd);
@@ -807,7 +809,9 @@ endOK:
   if (fd != NULL) {
     fclose(fd);
   }
-  estack_pop();
+  if (did_estack_push) {
+    estack_pop();
+  }
 
   return lp;
 }

src/nvim/testdir/test_autocmd.vim

@@ -2614,6 +2614,28 @@ func Test_BufWrite_lockmarks()
   call delete('Xtest2')
 endfunc
 
+func Test_FileType_spell()
+  if !isdirectory('/tmp')
+    throw "Skipped: requires /tmp directory"
+  endif
+
+  " this was crashing with an invalid free()
+  setglobal spellfile=/tmp/en.utf-8.add
+  augroup crash
+    autocmd!
+    autocmd BufNewFile,BufReadPost crashfile setf somefiletype
+    autocmd BufNewFile,BufReadPost crashfile set ft=anotherfiletype
+    autocmd FileType anotherfiletype setlocal spell
+  augroup END
+  func! NoCrash() abort
+    edit /tmp/crashfile
+  endfunc
+  call NoCrash()
+
+  au! crash
+  setglobal spellfile=
+endfunc
+
 " Test closing a window or editing another buffer from a FileChangedRO handler
 " in a readonly buffer
 func Test_FileChangedRO_winclose()