Add workaround for index out of bounds on escaping substs

[dario23]

See #188 for some discussion. The proper fix would probably be a bigger
refactoring endeavor, but this avoids the index out of bounds access for
now.

[JohnTitor]

The failure is:

rust-semverver failed to produce the expected result
=== Expected output ===
version bump: 1.0.0 -> (patch) -> 1.0.1
=== End of expected output ===
=== Actual output ===
version bump: 1.0.0 -> (breaking) -> 2.0.0
error: breaking changes in `b`
  --> infer/new.rs:13:1
   |
13 | pub fn b() -> a::Opq {
   | ^^^^^^^^^^^^^^^^^^^^
   |
   = warning: type error: expected struct `old::a::Klm`, found struct `new::a::Opq` (breaking)

error: aborting due to previous error
=== End of actual output ===

We now catch one more case? Nice.

[JohnTitor]

Closing as we're going to deprecate this crate, thank you for suggesting the change anyway!