Correctly initialize compressor_level_ to -1.

Otherwise, we would be of the mistaken impression that a compression
level had been set to 0, which is not what the code assumes to be the
default.  Probably due to a mismerge or simple braino at some point
in the past.

Reported by "Mack" on the WANProxy mailing list.
Merge branch 'master' of github.com:wanproxy/wanproxy