server: avoid the tcl server crashing when there is no target

Since commit 1d0cf0df37
("server: tcl_notifications command") connecting to the tcl server
would terminate openocd. Fix this.

Change-Id: I36e2a7482f7db3a30ff7e9f969c3b6cda9599382
Signed-off-by: Austin Morton <austinpmorton@gmail.com>
Reviewed-on: http://openocd.zylin.com/2759
Tested-by: jenkins
Reviewed-by: Forest Crossman <cyrozap@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

src/target/target.h

@@ -356,6 +356,7 @@ int target_call_timer_callbacks(void);
  */
 int target_call_timer_callbacks_now(void);
 
+struct target *get_target_by_num(int num);
 struct target *get_current_target(struct command_context *cmd_ctx);
 struct target *get_target(const char *id);