Fix bug discovered by valgrind.
Fix bug discovered by valgrind.

When trying to merge lines, we should not touch lines that are invalid,
as we don't know how long they might be.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@173043 91177308-0d34-0410-b5e6-96231b3b80d8

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