ceph_mon: don't dereference uninitialized pointer

ceph version 0.60-803-g7544e86 (7544e8663225903703e1d30baa8a7cbfcf771e1a)
 1: /usr/bin/ceph-mon() [0x596faa]
 2: (()+0xfcb0) [0x7f41cdfeccb0]
 3: (main()+0xda2) [0x48bba2]
 4: (__libc_start_main()+0xed) [0x7f41cc4c276d]
 5: /usr/bin/ceph-mon() [0x48eb1d]


Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>