fixed memory leak in Boolector.
fixed memory leak in Boolector.

All BtorExp* are remembered and released when the context is freed.

agra-uni-bremen/metaSMT,hoangmle/metaSMT,hoangmle/metaSMT,agra-uni-bremen/metaSMT,nbruns1/metaSMT,nbruns1/metaSMT,finnhaedicke/metaSMT,hoangmle/metaSMT,nbruns1/metaSMT,finnhaedicke/metaSMT,agra-uni-bremen/metaSMT,finnhaedicke/metaSMT