patch 7.4.1451

Problem:    Vim hangs when a channel has a callback but isn't referenced.
Solution:   Have channel_unref() only return TRUE when the channel was
            actually freed.

src/version.c

@@ -743,6 +743,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1451,
 /**/
     1450,
 /**/