These crashes were caused by an invalid pointer stored in a tagged
variable in SubStringStub. This can be reproduced by calling the stub on
an external string and ensuring GC kicks in on the subsequent
allocation.

Only the TurboFan implementation of SubStringStub is affected, the current
PlatformStub implementation handles this case just fine.

BUG=chromium:649967

Review-Url: https://codereview.chromium.org/2374603003
Cr-Commit-Position: refs/heads/master@{#39772}