patch 7.4.1421

Problem:    May free a channel when a callback may need to be invoked.
Solution:   Keep the channel when refcount is zero.

src/version.c

@@ -748,6 +748,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1421,
 /**/
     1420,
 /**/