Capacity decrease segfaults in non-released preemptive resources

Test case:

``` r
library(simmer)

t <- create_trajectory() %>%
  seize("dummy", 1)

sched <- schedule(c(0, 1), c(1, 0), period = Inf)

env <- simmer(verbose = TRUE) %>%
  add_resource("dummy", sched, preemptive = TRUE) %>%
  add_generator("arrival", t, at(0))

env %>% run()
```
