initialize decoded_internal_key_valid

Summary:
ReadInternalKey() will assign correct value anyway. Initialize it to
true to suppress compiler error reported
https://github.com/facebook/rocksdb/issues/186

Test Plan: I cannot reproduce it but this is obvious

Reviewers: sdong, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19467