patch 9.1.0855: setting 'cmdheight' may cause hit-enter-prompt

Problem:  setting 'cmdheight' may cause hit-enter-prompt and echo output
          to be missing
Solution: Before cleaning the cmdline, check the need_wait_return flag
          (nwounkn)

closes: #13432

Signed-off-by: nwounkn <nwounkn@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/window.c b/src/window.c
index 7c8f4fb..f8a6907 100644
--- a/src/window.c
+++ b/src/window.c
@@ -7343,11 +7343,14 @@
 	    // Recompute window positions.
 	    (void)win_comp_pos();
 
-	    // clear the lines added to cmdline
-	    if (full_screen)
-		screen_fill(cmdline_row, (int)Rows, 0,
-						   (int)Columns, ' ', ' ', 0);
-	    msg_row = cmdline_row;
+	    if (!need_wait_return)
+	    {
+		// clear the lines added to cmdline
+		if (full_screen)
+		    screen_fill(cmdline_row, (int)Rows, 0,
+						       (int)Columns, ' ', ' ', 0);
+		msg_row = cmdline_row;
+	    }
 	    redraw_cmdline = TRUE;
 	    return;
 	}