[SYCL][NFCI] Address static analyzer concerns

[AlexeySachkov]

Two changes were made:

Special padding field in accessors is now zero-initialized. We construct a host_accessor object in our stream implementation and static analyzer complained that some of the accessor's fields were uninitialized. It is not like we even reference padding field from anywhere, but the code change seems small, so I decided to add the initializer.

Fixed a condition to prevent access to an array of N elements at index N, which is outside of the array because they are 0-indexed.

[maarquitos14]

Just a nit.