librbd: assert on m_length in calc_object_diff_range()

A non-empty byte range is required by both last_period_off calculation
and Striper::file_to_extents().

Signed-off-by: Ilya Dryomov <[email protected]>

Author: idryomov

src/librbd/api/DiffIterate.cc

@@ -266,6 +266,7 @@ int DiffIterate<I>::diff_iterate(I *ictx,
 
 template <typename I>
 std::pair<uint64_t, uint64_t> DiffIterate<I>::calc_object_diff_range() {
+  ceph_assert(m_length > 0);
   uint64_t period = m_image_ctx.get_stripe_period();
   uint64_t first_period_off = round_down_to(m_offset, period);
   uint64_t last_period_off = round_down_to(m_offset + m_length - 1, period);