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().

src/screen.c

@@ -648,6 +648,8 @@ update_screen(int type_arg)
     {
 	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) */