Signs with invalid UTF-8 characters in the `Text` NBT causing client to lock up

Signs with some special (possibly corrupted) characters on them will cause the client to freeze and possibly crash.

For example, if the data `CgpQCqA35sI=` (base64-encoded because it contains non-printable characters) appears in a sign's `Text` NBT tag, the client will freeze and be unable to do anything except moving around.

This bug was discovered on a world created by a third party server.

I have verified that this bug does not happen in 1.8, this bug is 1.9-specific.