mon,osd: always init local variable

mon,osd: always init local variable

this change silences the warning like:

../src/mon/Monitor.cc: In member function ‘void Monitor::handle_command(MonOpRequestRef)’:
../src/mon/Monitor.cc:3703:55: warning: ‘osd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 3703 |     uint64_t seq = mgrstatmon()->get_last_osd_stat_seq(osd);
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>