bugfix: don't set pool_live_bytes to zero at the end of GC

[d-netto]

Sanity-checked this on a micro-benchmark (as I should have done two PRs ago) and seems to work now:

using Printf

mutable struct ListNode
  key::Int64
  next::ListNode
  ListNode() = new()
  ListNode(x)= new(x)
  ListNode(x,y) = new(x,y);
end

function list(n=128)
    start::ListNode = ListNode(1)
    current::ListNode = start
    for i = 2:(n*1024^2)
        current = ListNode(i,current)
    end
    @printf "%d\n" @ccall jl_gc_pool_live_bytes()::Int64
    return current.key
end

@time list()

4329322656
  3.560094 seconds (134.31 M allocations: 4.005 GiB, 81.34% gc time, 0.88% compilation time)