Fixed a bug in buffer_segment_reserve() which cause segv when buffer_split() occured.
Fixed a bug in buffer_segment_reserve() which cause segv when buffer_split() occured.

naoa/groonga,myokoym/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,naoa/groonga,redfigure/groonga,naoa/groonga,cosmo0920/groonga,kenhys/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,naoa/groonga,hiroyuki-sato/groonga,naoa/groonga,redfigure/groonga,kenhys/groonga,hiroyuki-sato/groonga,groonga/groonga,komainu8/groonga,hiroyuki-sato/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga,cosmo0920/groonga,cosmo0920/groonga,cosmo0920/groonga,redfigure/groonga,redfigure/groonga,redfigure/groonga,cosmo0920/groonga,kenhys/groonga,redfigure/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,hiroyuki-sato/groonga,myokoym/groonga,hiroyuki-sato/groonga,myokoym/groonga,myokoym/groonga,naoa/groonga,groonga/groonga,redfigure/groonga,redfigure/groonga,cosmo0920/groonga,myokoym/groonga,myokoym/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,cosmo0920/groonga,myokoym/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,hiroyuki-sato/groonga,myokoym/groonga,groonga/groonga