patch 7.4.1827

Problem:    No error when invoking a callback when it's not safe.
Solution:   Add an error message.  Avoid the error when freeing a channel.

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1827,
 /**/
     1826,
 /**/