debug assertion ut_ad(len > 0) with ut_ad(len >= OS_FILE_LOG_BLOCK_SIZE).

This change is only for readability, for Issue #428.  Another
assertion on len being an integer multiple of OS_FILE_LOG_BLOCK_SIZE
already ensured together with the old ut_ad(len > 0) that actually len
must be at least OS_FILE_LOG_BLOCK_SIZE.