target: check target supports target_request_data

Make sure the target support target requests before we enable any receivers.

Change-Id: I8ce42922eaff76fb5e7a114da716f2a6585a6ab5
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1536
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

src/target/target_request.c

@@ -122,6 +122,8 @@ int target_request(struct target *target, uint32_t request)
 {
 	target_req_cmd_t target_req_cmd = request & 0xff;
 
+	assert(target->type->target_request_data);
+
 	/* Record that we got a target message for back-off algorithm */
 	got_message = true;
 
@@ -256,6 +258,11 @@ COMMAND_HANDLER(handle_target_request_debugmsgs_command)
 
 	int receiving = 0;
 
+	if (target->type->target_request_data == NULL) {
+		LOG_ERROR("Target %s does not support target requests", target_name(target));
+		return ERROR_OK;
+	}
+
 	/* see if reciever is already registered */
 	if (find_debug_msg_receiver(CMD_CTX, target) != NULL)
 		receiving = 1;