librbd.cc: add check to prevent null deref in rbd_list()

Fix for :

CID 1192467 (2 of 2): Explicit null dereferenced (FORWARD_NULL)
 var_deref_model: Passing null pointer ibuf to rbd_list,
 which dereferences it.

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