patch 8.1.0905: complicated regexp causes a crash

Problem:    Complicated regexp causes a crash. (Kuang-che Wu)
Solution:   Limit the recursiveness of addstate(). (closes #3941)

src/testdir/test_regexp_latin.vim

@@ -84,3 +84,9 @@ func Test_multi_failure()
   call assert_fails('/a\{a}', 'E870:')
   set re=0
 endfunc
+
+func Test_recursive_addstate()
+  " This will call addstate() recursively until it runs into the limit.
+  let lnum = search('\v((){328}){389}')
+  call assert_equal(0, lnum)
+endfunc