Somehow search_n never got tested, so of course it had a bug in it.  This fixes http://llvm.org/bugs/show_bug.cgi?id=15667.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178764 91177308-0d34-0410-b5e6-96231b3b80d8