patch 8.2.4418: crash when using special multi-byte character

Problem:    Crash when using special multi-byte character.
Solution:   Don't use isalpha() for an arbitrary character.
diff --git a/src/testdir/test_autochdir.vim b/src/testdir/test_autochdir.vim
index 5d36775..95004d2 100644
--- a/src/testdir/test_autochdir.vim
+++ b/src/testdir/test_autochdir.vim
@@ -110,4 +110,12 @@
   call delete('Xautodir', 'rf')
 endfunc
 
+func Test_multibyte()
+  " using an invalid character should not cause a crash
+  set wic
+  call assert_fails('tc û¦*', 'E344:')
+  set nowic
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab