MIPS: Fix wrong asserts in SeqStringSetCharGenerator.

This commit fixes wrong asserts in commit r13230 (1929bb68).

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/11943003
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13427 ce2b1a6d-e550-0410-aec6-3dcde31c8c00