Fix no-return-in-nonvoid-function meas_vis.c

RPM post-build-checks found some issue and marks these as error:

[   38s] I: Program returns random data in a function
[   38s] E: openbsc no-return-in-nonvoid-function meas_vis.c:118

openbsc/src/utils/meas_vis.c

@@ -115,6 +115,8 @@ static int handle_msg(struct msgb *msg)
 	default:
 		break;
 	}
+
+	return 0;
 }
 
 static int udp_fd_cb(struct osmo_fd *ofd, unsigned int what)