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/version.c b/src/version.c
index fe100c8..9e77abc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -732,6 +732,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    240,
+/**/
     239,
 /**/
     238,