patch 8.2.3824: no ASAN support for MSVC

Problem:    No ASAN support for MSVC.
Solution:   Add ASAN support and fix a coupld of uncovered problems. (Yegappan
            Lakshmanan, closes #9357)
diff --git a/src/testdir/test_fnamemodify.vim b/src/testdir/test_fnamemodify.vim
index 7882f3c..dfe322a 100644
--- a/src/testdir/test_fnamemodify.vim
+++ b/src/testdir/test_fnamemodify.vim
@@ -93,6 +93,7 @@
   call assert_equal('b.c', fnamemodify('a.b.c.d.e', ':r:r:e:e:e'))
   call assert_equal('b.c', fnamemodify('a.b.c.d.e', ':r:r:e:e:e:e'))
 
+  call assert_equal('', fnamemodify('', ':p:t'))
   call assert_equal('', fnamemodify(test_null_string(), test_null_string()))
 endfunc