patch 8.1.0907: CI tests on AppVeyor are failing

Problem:    CI tests on AppVeyor are failing.
Solution:   Reduce the recursiveness limit for regexp.

src/regexp_nfa.c

@@ -4315,7 +4315,7 @@ addstate(
 
     // This function is called recursively.  When the depth is too much we run
     // out of stack and crash, limit recursiveness here.
-    if (++depth >= 10000 || subs == NULL)
+    if (++depth >= 5000 || subs == NULL)
     {
 	--depth;
 	return NULL;