patch 8.2.1748: closing split window in other tab may cause a crash

Problem:    Closing split window in other tab may cause a crash.
Solution:   Set tp_curwin properly. (Rob Pilling, closes #7018)
diff --git a/src/window.c b/src/window.c
index 75e0495..c73062d 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2745,6 +2745,7 @@
 {
     frame_T	*frp;
     win_T	*wp;
+    tabpage_T	*win_tp = tp == NULL ? curtab : tp;
 
     // Remove the window and its frame from the tree of frames.
     frp = win->w_frame;
@@ -2752,10 +2753,10 @@
     vim_free(frp);
     win_free(win, tp);
 
-    // When deleting the current window of another tab page select a new
-    // current window.
-    if (tp != NULL && win == tp->tp_curwin)
-	tp->tp_curwin = wp;
+    // When deleting the current window in the tab, select a new current
+    // window.
+    if (win == win_tp->tp_curwin)
+	win_tp->tp_curwin = wp;
 
     return wp;
 }