patch 8.0.0706: crash when cancelling the cmdline window in Ex mode

Problem:    Crash when cancelling the cmdline window in Ex mode. (James McCoy)
Solution:   Do not set cmdbuff to NULL, make it empty.
diff --git a/src/version.c b/src/version.c
index 93d26ef..33399a4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    706,
+/**/
     705,
 /**/
     704,