osmo_io: Ensure correct ownership of msgb when sending

Use talloc_steal() if a msg is passed in to osmo_io when sending. This
avoids the message being free()d early in case the original parent is
free()d.

Change-Id: Ie36bd68a8bd63e67d76fb41996f8fdf99f51d96c

src/core/osmo_io.c

@@ -102,6 +102,8 @@ struct iofd_msghdr *iofd_msghdr_alloc(struct osmo_io_fd *iofd, enum iofd_msg_act
 			talloc_free(hdr);
 			return NULL;
 		}
+	} else {
+		talloc_steal(iofd->msgb_alloc.ctx, msg);
 	}
 
 	hdr->action = action;