Changes

osmo_io: Add assert to validate inconditional

The do-while loop is entered:
* First time: "tail_msg != NULL" (since it was set to "msg").
* Subsequent times: coming from "while" condition:
** tail_msg != NULL
** res == IOFD_SEG_ACT_HANDLE_MORE, which means iofd->pending != NULL.

Hence, always either "iofd->pending" or "tail_msg" is valid when entering
the loop, and hence "msg" will also be valid.

Related: Coverity CID#557196
Change-Id: If29363edfba576370e306fb5b160d9f038018130