patch 8.1.0692: if a buffer was deleted a channel can't write to it

Problem:    If a buffer was deleted a channel can't write to it.
Solution:   When the buffer exists but was unloaded, prepare it for writing.
            (closes #3764)
diff --git a/src/version.c b/src/version.c
index 144c9ef..193e327 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    692,
+/**/
     691,
 /**/
     690,