fix: null-stride guard in canUse32BitIndexMath

utils.h

@@ -673,6 +673,10 @@ CUHOST inline bool canUse32BitIndexMath(
  if (numel >= max32) return false;
  if (numel == 0) return max32 > 0;
 
+ // A null stride array is DLPack's compact row-major tensor: the largest
+ // offset is numel-1, already known < max32 here, so 32-bit math is safe.
+ if (stride == nullptr) return true;
+
  int64_t offset = 0;
  int64_t lin_index = numel - 1;