patch 8.2.3409: reading beyond end of line with invalid utf-8 character

Problem:    Reading beyond end of line with invalid utf-8 character.
Solution:   Check for NUL when advancing.
diff --git a/src/testdir/test_regexp_utf8.vim b/src/testdir/test_regexp_utf8.vim
index 9f0ffb9..044aeff 100644
--- a/src/testdir/test_regexp_utf8.vim
+++ b/src/testdir/test_regexp_utf8.vim
@@ -558,4 +558,12 @@
   bwipe!
 endfunc
 
+func Test_match_invalid_byte()
+  call writefile(0z630a.765d30aa0a.2e0a.790a.4030, 'Xinvalid')
+  new
+  source Xinvalid
+  bwipe!
+  call delete('Xinvalid')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab