patch 8.0.0762: ml_get error with :psearch in buffer without a name

Problem:    ml_get error with :psearch in buffer without a name. (Dominique
            Pelle)
Solution:   Use the buffer number instead of the file name.  Check the cursor
            position.
diff --git a/src/testdir/test_preview.vim b/src/testdir/test_preview.vim
new file mode 100644
index 0000000..91923fb
--- /dev/null
+++ b/src/testdir/test_preview.vim
@@ -0,0 +1,13 @@
+" Tests for the preview window
+
+func Test_Psearch()
+  " this used to cause ml_get errors
+  help
+  let wincount = winnr('$')
+  0f
+  ps.
+  call assert_equal(wincount + 1, winnr('$'))
+  pclose
+  call assert_equal(wincount, winnr('$'))
+  bwipe
+endfunc