AdminSocket: initialize m_getdescs_hook in the constructor

CID 717212 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
 2. uninit_member: Non-static class member "m_getdescs_hook" is not
 initialized in this constructor nor in any functions that it calls.

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