patch 8.2.1729: endless loop when ":normal" feeds popup window filter

Problem:    Endless loop when ":normal" feeds popup window filter.
Solution:   Add the ex_normal_busy_done flag.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 570aef8..9bdeb52 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2610,7 +2610,13 @@
 		    ++ex_normal_busy;
 		exec_normal(TRUE, lowlevel, TRUE);
 		if (!dangerous)
+		{
 		    --ex_normal_busy;
+#ifdef FEAT_PROP_POPUP
+		    if (ex_normal_busy == 0)
+			ex_normal_busy_done = FALSE;
+#endif
+		}
 
 		msg_scroll |= save_msg_scroll;
 	    }