Fixed heap-buffer-overflow issue when Options.num_levels > 7.

Summary:
Currently, when num_levels has been changed to > 7, internally
it will not resize max_bytes_for_level_multiplier_additional.
As a result, max_bytes_for_level_multiplier_additional.size() will
be smaller than num_levels, which causes heap-buffer-overflow.

Test Plan: make all check

Reviewers: haobo, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19275