Unreleased semaphore blocks httprc indefinitely  if a Transform panics

## Description

The semaphore acquired here https://github.com/lestrrat-go/httprc/blob/e71784b6c22397cb2a0cf2579a4b90083c797fc0/cache.go#L155 isn't released with a `defer` 

If a `Transform` panics the semaphore is never released, and the next call to a `Cache.Get` will block indefinitely 
