Safety check for negative alloc_cpu() attempt (#17071)

Safety check for negative alloc_cpu() attempt (#17071)

Summary:
Some legacy TH code was relying on alloc to throw when called with negative number!!! E.g. `torch.linspace(0, 1, -1)`. And it breaks ASAN build. I still believe alloc should receive size_t, but I added a safety enforce inside.

It should fix ASAN. I'll follow up with a proper fix for empty_cpu (which is probably the right place to do it) separately
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17071

Differential Revision: D14074157

Pulled By: dzhulgakov

fbshipit-source-id: 3ed3bdb873e446edecb558e1df491310fd7179e3