rgw_rados.h: generator class: initialize manifest with NULL

CID 1188180 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "manifest" is not initialized
 in this constructor nor in any functions that it calls.

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