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)

src/terminal.c

@@ -2952,7 +2952,10 @@ handle_postponed_scrollback(term_T *term)
 {
     int i;
 
+    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);

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1018,
 /**/
     1017,
 /**/