patch 8.1.0661: clipboard regexp might be used recursively

Problem:    Clipboard regexp might be used recursively.
Solution:   Check for recursive use and bail out.
diff --git a/src/version.c b/src/version.c
index c98fc50..c49e869 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    661,
+/**/
     660,
 /**/
     659,