heap_realloc returned NULL when a block outgrew the largest size class or
the budget, and libuv aborts when growing its watcher array fails: a poll
on fd 8190 or above (about 4,000 connections) killed the process.
Move the block to the system heap instead, as malloc and calloc already
do. test_heap_guard_cpp polls fd 16380 with the heaps installed.
