Don't log/crash when PrefilterTree::KeepNode() sees Prefilter::NONE.
Don't log/crash when PrefilterTree::KeepNode() sees Prefilter::NONE.

Change-Id: I66c4d61419052e110d5b1ad6a62cd8d8a86f15a3
Reviewed-on: https://code-review.googlesource.com/c/34910
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2