Patches for the allocators (malloc/debug).

[Benjamin Kosnik]

>I don't know what the intention was in making malloc_allocator not throw
>anything but that makes it non-compliant, and also the user is not
>warned about any unsuccessful allocations, So I have added that check.

Yep. Good catch!

>Also, I have added another check in debug_allocator for checking whether
>a NULL pointer was passed.

Sounds good.

I apologize for the delay.

This patch is in, thanks!

-benjamin