patch 8.0.0849: crash when job exit callback wipes the terminal

Problem:    Crash when job exit callback wipes the terminal.
Solution:   Check for b_term to be NULL.  (Yasuhiro Matsumoto, closes #1922)
            Implement options for term_start() to be able to test.
            Make term_wait() more reliable.

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    849,
 /**/
     848,
 /**/