patch 8.0.1617: Win32: :shell command in the GUI crashes

Problem:    Win32: :shell command in the GUI crashes.
Solution:   Handle the situation that "cmd" is NULL. (Yasuhiro Matsumoto,
            closes #2721)
diff --git a/src/version.c b/src/version.c
index 58c4e89..92ca63a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1617,
+/**/
     1616,
 /**/
     1615,