req_state: fix uninitialized bool var

CID 717359 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
 uninit_member: Non-static class member "bucket_exists" is not
 initialized in this constructor nor in any functions that it calls.

Set bucket_exists to false in req_state::req_state().

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