Annotate return_overflow in mod_write() (CID #1604620)

This is arguably another example of trying to return a value not
representable in the function return type. It's highly unlikely that
anyone will pass a buffer of more than SSIZE_MAX bytes, but Coverity
apparently doesn't consider that.

CIDs #1604605 and #1604616 explicitly do return error values not
representable as int, but a ridiculously large buffer allocation will
fail long before anyone calls mod_write(), so we annotate.

src/listen/tacacs/proto_tacacs_tcp.c

@@ -315,6 +315,7 @@ static ssize_t mod_write(fr_listen_t *li, UNUSED void *packet_ctx, UNUSED fr_tim
 	 *	Return the packet we wrote, plus any bytes previously
 	 *	left over from previous packets.
 	 */
+	/* coverity[return_overflow] */
 	return data_size + written;
 }