patch 8.2.0117: crash when using gettabwinvar() with invalid arguments

Problem:    Crash when using gettabwinvar() with invalid arguments. (Yilin
            Yang)
Solution:   Use "curtab" if "tp" is NULL. (closes #5475)
diff --git a/src/evalwindow.c b/src/evalwindow.c
index 672bfc1..fb54033 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -183,7 +183,8 @@
     {
 #ifdef FEAT_PROP_POPUP
 	// check tab-local popup windows
-	for (wp = tp->tp_first_popupwin; wp != NULL; wp = wp->w_next)
+	for (wp = (tp == NULL ? curtab : tp)->tp_first_popupwin;
+						   wp != NULL; wp = wp->w_next)
 	    if (wp->w_id == nr)
 		return wp;
 	// check global popup windows