Mineshafts can intersect the bedrock layer

The Bug:

Mineshafts can generate in the bedrock layer. Relates to Bedrock bug ticket MCPE-184894.

Steps to Reproduce:

1. Create a world with this seed: -7213800522284939450
2. Go to the following coordinates: 5891 -60 6044
3. You will find a mineshaft generated in the bedrock:

Expected Results:

Mineshafts must not generate this near to the bedrock layer.

Observed Results:

Mineshafts can generate this near to the bedrock layer.