vim-patch:9.0.0614: SpellFileMissing autocmd may delete buffer

Problem:    SpellFileMissing autocmd may delete buffer.
Solution:   Disallow deleting the current buffer to avoid using freed memory.
ef976323e7

src/nvim/testdir/test_autocmd.vim

@@ -2752,6 +2752,16 @@ func Test_FileType_spell()
   setglobal spellfile=
 endfunc
 
+" this was wiping out the current buffer and using freed memory
+func Test_SpellFileMissing_bwipe()
+  next 0
+  au SpellFileMissing 0 bwipe
+  call assert_fails('set spell spelllang=0', 'E937:')
+
+  au! SpellFileMissing
+  bwipe
+endfunc
+
 " Test closing a window or editing another buffer from a FileChangedRO handler
 " in a readonly buffer
 func Test_FileChangedRO_winclose()