message: Make assertion of not freeing a null ref unconditional
diff: @@ -606,7 +606,7 @@ void SlabAlloc::do_free(ref_type ref, char* addr) noexcept
  Chunk chunk;
  chunk.ref = ref;
  chunk.size = size;
  - REALM_ASSERT(ref != 0);
  + REALM_ASSERT_RELEASE(ref != 0);
  m_free_read_only.push_back(chunk); // Throws
  }
  catch (...) {
  