patch 8.1.0425: ml_get error and crash with appendbufline()

Problem:    ml_get error and crash with appendbufline(). (Masashi Iizuka)
Solution:   Set per-window buffer info. (Hirohito Higashi, closes #3455)
diff --git a/src/buffer.c b/src/buffer.c
index b536be6..a4afe13 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -428,7 +428,10 @@
 
 	FOR_ALL_WINDOWS(wp)
 	    if (wp->w_buffer == buf)
+	    {
 		can_unload = FALSE;
+		break;
+	    }
     }
     if (!can_unload)
 	EMSG(_("E937: Attempt to delete a buffer that is in use"));
@@ -1742,6 +1745,9 @@
     /* mark cursor position as being invalid */
     curwin->w_valid = 0;
 
+    buflist_setfpos(curbuf, curwin, curbuf->b_last_cursor.lnum,
+					      curbuf->b_last_cursor.col, TRUE);
+
     /* Make sure the buffer is loaded. */
     if (curbuf->b_ml.ml_mfp == NULL)	/* need to load the file */
     {