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)

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    692,
 /**/
     691,
 /**/