fix segfault (because of erasing after the vector boundaries) when the cached token position is at the end
fix segfault (because of erasing after the vector boundaries) when the cached token position is at the end

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@77159 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang