patch 8.0.0355: using uninitialized memory when 'isfname' is empty

Problem:    Using uninitialized memory when 'isfname' is empty.
Solution:   Don't call getpwnam() without an argument. (Dominique Pelle,
            closes #1464)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 9ac46f2..11466dc 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -22,6 +22,13 @@
   set whichwrap&
 endfunction
 
+function! Test_isfname()
+  " This used to cause Vim to access uninitialized memory.
+  set isfname=
+  call assert_equal("~X", expand("~X"))
+  set isfname&
+endfunction
+
 function Test_options()
   let caught = 'ok'
   try