patch 8.1.0660: sign_cleanup() may leak memory

Problem:    sign_cleanup() may leak memory.
Solution:   Free the group name before returning.  Add a few more tests.
            (Yegappan Lakshmanan)
diff --git a/src/version.c b/src/version.c
index e76bf4b..c98fc50 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    660,
+/**/
     659,
 /**/
     658,