Fix --hydrogen-filter="-".

SubVector asserts that the vector length is not 0 and fails.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/43873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17405 ce2b1a6d-e550-0410-aec6-3dcde31c8c00