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 <[email protected]>
diff --git a/re2/prefilter_tree.cc b/re2/prefilter_tree.cc
index 376c6cd..a07de40 100644
--- a/re2/prefilter_tree.cc
+++ b/re2/prefilter_tree.cc
@@ -138,6 +138,7 @@
       return false;
 
     case Prefilter::ALL:
+    case Prefilter::NONE:
       return false;
 
     case Prefilter::ATOM: