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.

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    661,
 /**/
     660,
 /**/