alloc debug overflow on size 0

if the size is 0, this will abort on a sub overflow in debug mode.

```rust
fn alloc(
&mut self,
device: &B::Device,
size: u64,
align: u64,
) -> Result<(DynamicBlock, u64), gfx_hal::device::AllocationError> {
debug_assert!(size <= self.max_allocation());
debug_assert!(align.is_power_of_two());
let aligned_size = ((size - 1) | (align - 1) | (self.block_size_granularity - 1)) + 1;
```