patch 8.1.1720: crash with very long %[] pattern

Problem:    Crash with very long %[] pattern. (Reza Mirzazade farkhani)
Solution:   Check for reg_toolong. (closes #4703)
diff --git a/src/testdir/test_regexp_utf8.vim b/src/testdir/test_regexp_utf8.vim
index 98b9e73..19ff882 100644
--- a/src/testdir/test_regexp_utf8.vim
+++ b/src/testdir/test_regexp_utf8.vim
@@ -206,3 +206,12 @@
   call assert_equal(1, "\u3042" =~# '[\u3000-\u4000]')
   set re=0
 endfunc
+
+func Test_optmatch_toolong()
+  set re=1
+  " Can only handle about 8000 characters.
+  let pat = '\\%[' .. repeat('x', 9000) .. ']'
+  call assert_fails('call match("abc def", "' .. pat .. '")', 'E339:')
+  set re=0
+endfunc
+