Suppose the following happens:

vbp_task() finishes with vt->running = 0 and a heap insert. The
vbp_cond is signaled under the lock, but now instead of vbp_thread()
waking up first, VBP_Remove() gets the lock and reaches
assert(vt->heap_idx == VBH_NOIDX) before the racing vbp_thread()
deleted the heap.

This is unlikely to happen with static backends, because for those,
the probe is stopped via the vcl temperature before they get removed.

Fixes https://github.com/nigoroll/libvmod-dynamic/issues/100