rbd.cc: init 'snap_protected' to fix -Wconditional-uninitialized

rbd.cc: init 'snap_protected' to fix -Wconditional-uninitialized

Init 'snap_protected' with false to fix:

rbd.cc:544:35: warning: variable 'snap_protected' may be uninitialized
 when used here [-Wconditional-uninitialized]
      f->dump_string("protected", snap_protected ? "true" : "false");
                                  ^~~~~~~~~~~~~~
rbd.cc:482:22: note: initialize the variable 'snap_protected' to silence
 this warning
  bool snap_protected;
                     ^
                      = false

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