patch 8.0.1257: no test for fix of undefined behavior

Problem:    No test for fix of undefined behavior.
Solution:   Add a test. (closes #2255)
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index be8b9a4..37661c3 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -697,3 +697,7 @@
   call term_sendkeys(g:buf, ":qa!\<cr>")
   bwipe!
 endfunc
+
+func Test_search_undefined_behaviour2()
+  call search("\%UC0000000")
+endfunc