common: bufferlist::get_contiguous return 0 when param len == 0

Change-Id: I3eaee45b12c634687cd1b294b3c264aed8cab03a
Signed-off-by: Dong Yuan <yuandong1222@gmail.com>
Merge pull request #3094 from yuandong1222/bufferlist-get_contiguous

common: bufferlist::get_contiguous return 0 when param len == 0

Reviewed-by: Sage Weil <sage@redhat.com>