vim-patch:8.2.4440: crash with specific regexp pattern and string

Problem:    Crash with specific regexp pattern and string.
Solution:   Stop at the start of the string.
6456fae9ba

src/nvim/testdir/test_regexp_utf8.vim

@@ -540,7 +540,6 @@ endfunc
 " Check that [[:upper:]] matches for automatic engine
 func Test_match_char_class_upper()
   new
-  let _engine=&regexpengine
 
   " Test 1: [[:upper:]]\{2,\}
   set regexpengine=0
@@ -581,7 +580,7 @@ func Test_match_char_class_upper()
   call assert_equal(4, searchcount().total, 'TEST 3 lower')
 
   " clean up
-  let &regexpengine=_engine
+  set regexpengine=0
   bwipe!
 endfunc
 
@@ -593,4 +592,13 @@ func Test_match_invalid_byte()
   call delete('Xinvalid')
 endfunc
 
+func Test_match_too_complicated()
+  set regexpengine=1
+  exe "vsplit \xeb\xdb\x99"
+  silent! buf \&\zs*\zs*0
+  bwipe!
+  set regexpengine=0
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab