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.

src/spell.c

@@ -1568,6 +1568,10 @@ spell_load_lang(char_u *lang)
     sl.sl_slang = NULL;
     sl.sl_nobreak = FALSE;
 
+    // Disallow deleting the current buffer.  Autocommands can do weird things
+    // and cause "lang" to be freed.
+    ++curbuf->b_locked;
+
     // We may retry when no spell file is found for the language, an
     // autocommand may load it then.
     for (round = 1; round <= 2; ++round)
@@ -1621,6 +1625,8 @@ spell_load_lang(char_u *lang)
 	STRCPY(fname_enc + STRLEN(fname_enc) - 3, "add.spl");
 	do_in_runtimepath(fname_enc, DIP_ALL, spell_load_cb, &sl);
     }
+
+    --curbuf->b_locked;
 }
 
 /*