sml_trimstore() already delays freeing of a replaced last, smaller segment for
the reallocation case at the bottom of the function: Because a concurrently
running iterator might have already taken a reference on a to-be-replaced
segment, it can not be freed immediately, but rather is kept around until the
busy object is no more.

This trivial change applies the same also for a segment which turns out to be
unneeded because writing the object ended with a zero length in this segment.

Simple, but consequential, see next commit