Generating nether portal can break bedrock

Generating a new nether portal by going through one from another dimension destroys bedrock when there are no other blocks for the portal to generate in the target dimension. One possible solution would be to put a limit on how low portals can generate to avoid breaking naturally generated bedrock.

Steps to reproduce:

1. Create a 1 block thick bedrock superflat world
2. Go into the Nether
3. Go 100+ blocks away and create another Nether portal back to overworld.
4. Enter the Nether portal.

Observed results:
The generated Nether portal replaces some of the bedrock in the target dimension.

Expected results:
The generated Nether portal does not replace some of the bedrock in the target dimension.