patch 8.1.1018: window cleared when entering Terminal-Normal twice

Problem:    Window cleared when entering Terminal-Normal twice. (Epheien)
Solution:   Don't cleanup scrollback when there is no postponed scrollback.
            (Christian Brabandt, closes #4126)
diff --git a/src/terminal.c b/src/terminal.c
index 8d05a18..6b16779 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -2952,7 +2952,10 @@
 {
     int i;
 
-ch_log(NULL, "Moving postponed scrollback to scrollback");
+    if (term->tl_scrollback_postponed.ga_len == 0)
+	return;
+    ch_log(NULL, "Moving postponed scrollback to scrollback");
+
     // First remove the lines that were appended before, the pushed lines go
     // above it.
     cleanup_scrollback(term);