Add boundary check for ver2 bigram reading.

...suggest/core/policy/dictionary_bigrams_structure_policy.h

.../structure/backward/v402/bigram/ver4_bigram_list_policy.h

...ructure/pt_common/bigram/bigram_list_read_write_utils.cpp

...structure/pt_common/bigram/bigram_list_read_write_utils.h

...yimpl/dictionary/structure/v2/bigram/bigram_list_policy.h

...licyimpl/dictionary/structure/v2/patricia_trie_policy.cpp

...policyimpl/dictionary/structure/v2/patricia_trie_policy.h

.../dictionary/structure/v4/bigram/ver4_bigram_list_policy.h