Remove unreachable lines in sampling code; test model logp before sampling

This is an auto-generated issue, triggered by the even-with-pymc3 workflow.

There have been new commits to pymc3/sampling.py or pymc3/step_methods/hmc/ since yesterday: please see the latest run of the even-with-pymc3 GitHub Action and consider if these commits are worth mirroring in littlemcmc.

Upstream PRs: https://github.com/pymc-devs/pymc3/pull/4261 and https://github.com/pymc-devs/pymc3/pull/4211

Both seem relevant to LittleMCMC.