Summary:
The db_bench utility was broken in 1.5.4.fb because of a
signed-unsigned comparision.

The static variable FLAGS_min_level_to_compress was recently
changed from int to 'unsigned in' but it is initilized to a
nagative value -1.

The segfault is of this type:
Program received signal SIGSEGV, Segmentation fault.
Open (this=0x7fffffffdee0) at db/db_bench.cc:939
939	db/db_bench.cc: No such file or directory.
(gdb) where

Test Plan: run db_bench with no options.

Reviewers: heyongqiang

Reviewed By: heyongqiang

CC: MarkCallaghan, emayanke, sheki

Differential Revision: https://reviews.facebook.net/D6663