appwiz.cpl: Correctly handle NULL params in CPL_STARTWPARAMSW handler.

dlls/appwiz.cpl/appwiz.c

@@ -958,6 +958,9 @@ static LONG start_params(const WCHAR *params)
 {
     static const WCHAR install_geckoW[] = {'i','n','s','t','a','l','l','_','g','e','c','k','o',0};
 
+    if(!params)
+        return FALSE;
+
     if(!strcmpW(params, install_geckoW)) {
         install_wine_gecko();
         return TRUE;