fix memory leak in MultipleColumnKeyCodec::encode_blob() refs #1958
fix memory leak in MultipleColumnKeyCodec::encode_blob() refs #1958

mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga