TailDuplicator: Fix crash after r278974
TailDuplicator: Fix crash after r278974

Some inputs would after r278974 without this fix (see
http://lab.llvm.org:8080/green/job/clang-stage2-cmake-RgSan_build/2733/console
for an example)

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@279022 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit b1cfb8c59e467c5333c0f5c67dd8ef1152bf3e4e)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm