patch 9.0.0240: crash when using ":mkspell" with an empty .dic file

Problem:    Crash when using ":mkspell" with an empty .dic file.
Solution:   Check for an empty word tree.
diff --git a/src/testdir/test_spellfile.vim b/src/testdir/test_spellfile.vim
index 43dfad4..28e69bd 100644
--- a/src/testdir/test_spellfile.vim
+++ b/src/testdir/test_spellfile.vim
@@ -1168,4 +1168,16 @@
   delete('Xtest', 'rf')
 enddef
 
+" this was using a NULL pointer
+func Test_mkspell_empty_dic()
+  call writefile(['1'], 'XtestEmpty.dic')
+  call writefile(['SOFOFROM abcd', 'SOFOTO ABCD', 'SAL CIA X'], 'XtestEmpty.aff')
+  mkspell! XtestEmpty.spl XtestEmpty
+
+  call delete('XtestEmpty.dic')
+  call delete('XtestEmpty.aff')
+  call delete('XtestEmpty.spl')
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab