patch 8.1.0960: when using ConPTY garbage collection has undefined behavior

Problem:    When using ConPTY garbage collection has undefined behavior.
Solution:   Free the channel in a better way. (Nobuhiro Takasaki, closes #4020)
diff --git a/src/version.c b/src/version.c
index 0d9acf4..9157ff5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    960,
+/**/
     959,
 /**/
     958,