Fix signed/unsigned comparison statements.

When a network interface of a path is down, bandwith of the concerned peer isn't published with a value tending toward zero as expected. It's caused by a signed/unsigned comparison.

As rist_send_seq_rtcp relies on sendmsg and sendto it can return only -1 on error as mentionned by their man pages.

Consequently, checking errors can be implemented with this statement: if (ret == (size_t)-1)