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.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    596,
 /**/
     595,
 /**/