Fixed UB in a test caught by UBSan, switch to unsigned arithmetic
Fixes: TestImageStencilIterator.cxx:35:27: runtime error: signed integer overflow: 1664525 * 230981 cannot be represented in type 'int'
Fixes: TestImageStencilIterator.cxx:35:27: runtime error: signed integer overflow: 1664525 * 230981 cannot be represented in type 'int'