The problem is that a MAP_FAILED segment ends up in its vsm_set::segs
list, at which point NULL checks will determine what to do with the
segment, and MAP_FAILED != NULL.

This can happen when winning (or losing?) the race with a VSC segment
that goes away after being found in the index, but before being mapped.

One surefire result is a crash in vsm_unmapseg() tripping on assertions
that other fields are also not NULL. There may be other unidentified ill
effects.