RadosModel.h: initialize CopyFromOp::snap

CID 1093219 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
 4. uninit_member: Non-static class member "snap" is not initialized
 in this constructor nor in any functions that it calls.

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