patch 8.0.0365: might free a dict item that wasn't allocated

Problem:    Might free a dict item that wasn't allocated.
Solution:   Call dictitem_free(). (Nikolai Pavlov)  Use this for
            b:changedtick.
diff --git a/src/fileio.c b/src/fileio.c
index 73baecc..2001c0d 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4924,11 +4924,11 @@
     {
 	unchanged(buf, TRUE);
 #ifdef FEAT_AUTOCMD
-	/* buf->b_changedtick is always incremented in unchanged() but that
+	/* b:changedtick is always incremented in unchanged() but that
 	 * should not trigger a TextChanged event. */
-	if (last_changedtick + 1 == *buf->b_changedtick
+	if (last_changedtick + 1 == CHANGEDTICK(buf)
 					       && last_changedtick_buf == buf)
-	    last_changedtick = *buf->b_changedtick;
+	    last_changedtick = CHANGEDTICK(buf);
 #endif
 	u_unchanged(buf);
 	u_update_save_nr(buf);