macos::OsIpcSharedMemory::from_bytes aborts if passed an empty slice

[jdm]

* thread #43, stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x000000011ba94c29 servo`core::intrinsics::copy_nonoverlapping::h64863016c6ffb52b(src="", dst=0x0000000000000000, count=0) at intrinsics.rs:2125:9
   2122	            && is_nonoverlapping(src, dst, count))
   2123	    {
   2124	        // Not panicking to keep codegen impact smaller.
-> 2125	        abort();
   2126	    }
   2127
   2128	    // SAFETY: the safety contract for `copy_nonoverlapping` must be
Target 0: (servo) stopped.
(lldb) li -
   2115	    extern "rust-intrinsic" {
   2116	        fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
   2117	    }
   2118
   2119	    if cfg!(debug_assertions)
   2120	        && !(is_aligned_and_not_null(src)
   2121	            && is_aligned_and_not_null(dst)
(lldb) up
frame #1: 0x000000011b2685cf servo`ipc_channel::platform::macos::OsIpcSharedMemory::from_bytes::h04f07f4eb147b3e9(bytes=(data_ptr = "", length = 0)) at mod.rs:900:13
   897 	    pub fn from_bytes(bytes: &[u8]) -> OsIpcSharedMemory {
   898 	        unsafe {
   899 	            let address = allocate_vm_pages(bytes.len());
-> 900 	            ptr::copy_nonoverlapping(bytes.as_ptr(), address, bytes.len());
   901 	            OsIpcSharedMemory::from_raw_parts(address, bytes.len())
   902 	        }
   903 	    }
(lldb) up
frame #2: 0x000000011b2197bd servo`ipc_channel::ipc::IpcSharedMemory::from_bytes::h23c8b06c54dcec83(bytes=(data_ptr = "", length = 0)) at ipc.rs:509:31
   506 	    /// Create shared memory initialized with the bytes provided.
   507 	    pub fn from_bytes(bytes: &[u8]) -> IpcSharedMemory {
   508 	        IpcSharedMemory {
-> 509 	            os_shared_memory: OsIpcSharedMemory::from_bytes(bytes),
   510 	        }
   511 	    }
   512

The root here is that allocate_vm_pages can return a null pointer. We should probably make it return Option<NonNull<u8> instead and force callers to deal with that appropriately.