memory-model: correctly handle result of libc::mmap()
memory-model: correctly handle result of libc::mmap()

Function libc::mmap() returns libc::MAP_FAILED(-1) instead of null on
failures, so don't detect failure by checking that the returned pointer
is null.

Signed-off-by: Liu Jiang <aaf6961e30ae80fd3a577c4cb6742ab6a1ad13eb@linux.alibaba.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker