patch 8.0.0686: extra redraw when using CTRL-L in second window

Problem:    When typing CTRL-L in a window that's not the first one, another
            redraw will happen later. (Christian Brabandt)
Solution:   Reset must_redraw after calling screenclear().
diff --git a/src/screen.c b/src/screen.c
index 3b281a8..f047534 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -648,6 +648,8 @@
     {
 	screenclear();		/* will reset clear_cmdline */
 	type = NOT_VALID;
+	/* must_redraw may be set indirectly, avoid another redraw later */
+	must_redraw = 0;
     }
 
     if (clear_cmdline)		/* going to clear cmdline (done below) */