Fix arena_size computation.

Fix arena_size arena_new() computation to incorporate
runs_avail_nclasses elements for runs_avail, rather than
(runs_avail_nclasses - 1) elements.  Since offsetof(arena_t, runs_avail)
is used rather than sizeof(arena_t) for the first term of the
computation, all of the runs_avail elements must be added into the
second term.

This bug was introduced (by Jason Evans) while merging pull request #330
as 3417a304 (Separate arena_avail
trees).