patch 8.0.0797: finished job in terminal window is not handled

Problem:    Finished job in terminal window is not handled.
Solution:   Add the scrollback buffer.  Use it to fill the buffer when the job
            has ended.

src/buffer.c

@@ -858,7 +858,7 @@ free_buffer(buf_T *buf)
     channel_buffer_free(buf);
 #endif
 #ifdef FEAT_TERMINAL
-    free_terminal(buf->b_term);
+    free_terminal(buf);
 #endif
 
     buf_hashtab_remove(buf);