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/regexp.c

@@ -8209,6 +8209,15 @@ report_re_switch(char_u *pat)
 }
 #endif
 
+/*
+ * Return whether "prog" is currently being executed.
+ */
+    int
+regprog_in_use(regprog_T *prog)
+{
+    return prog->re_in_use;
+}
+
 /*
  * Match a regexp against a string.
  * "rmp->regprog" is a compiled regexp as returned by vim_regcomp().