patch 8.2.1539: using invalid script ID causes a crash

Problem:    Using invalid script ID causes a crash.
Solution:   Check the script ID to be valid. (closes #6804)
diff --git a/src/scriptfile.c b/src/scriptfile.c
index 27d0eb2..e075236 100644
--- a/src/scriptfile.c
+++ b/src/scriptfile.c
@@ -1517,7 +1517,7 @@
     if (eap->addr_count > 0)
     {
 	// :script {scriptId}: edit the script
-	if (eap->line2 < 1 || eap->line2 > script_items.ga_len)
+	if (!SCRIPT_ID_VALID(eap->line2))
 	    emsg(_(e_invarg));
 	else
 	{