[XPTI] Add assert to prevent unintentional integer underflow.

[uditagarwal97]

Coverity flagged this.
Fixes #18513

[uditagarwal97]

Failure in Windows BMG:

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Failed Tests (3):
  SYCL :: Adapters/level_zero/ext_intel_cslice.cpp
  SYCL :: Adapters/level_zero/ext_intel_queue_index.cpp
  SYCL :: Adapters/sycl-ls-uuid-subdevs.cpp

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Unexpectedly Passed Tests (5):
  SYCL :: bindless_images/array/fetch_handle_carray2d.cpp
  SYCL :: bindless_images/read_1D_O0.cpp
  SYCL :: bindless_images/sampled_fetch/fetch_1D_O0.cpp
  SYCL :: bindless_images/sampled_fetch/fetch_2D_O0.cpp
  SYCL :: bindless_images/sampling_1D_O0.cpp

Adapter tests were marked XFAIL in #19242 and Bindless images tests were fixed in #19215. All of these test failures are unrelated to this PR. Merging.