patch 8.0.0596: crash when complete() called after complete_add()

Problem:    Crash when complete() is called after complete_add() in
            'completefunc'. (Lifepillar)
Solution:   Bail out if compl_pattern is NULL. (closes #1668)
            Also avoid using freed memory.
diff --git a/src/version.c b/src/version.c
index 8895b73..f31a946 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    596,
+/**/
     595,
 /**/
     594,