patch 8.0.1373: no error when settting 'renderoptions' before starting GUI

Problem:    No error when settting 'renderoptions' to an invalid value before
            starting the GUI.
Solution:   Always check the value. (Ken Takata, closes #2413)
diff --git a/src/gui_w32.c b/src/gui_w32.c
index c3178ba..119bcd5 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -127,6 +127,9 @@
 	    return FAIL;
     }
 
+    if (!gui.in_use)
+	return OK;  /* only checking the syntax of the value */
+
     /* Enable DirectX/DirectWrite */
     if (dx_enable)
     {