AbstractWrite: initialize m_snap_seq with 0

CID 717223 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "m_snap_seq" is not initialized
 in this constructor nor in any functions that it calls.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>