Fix edge case in update pending deal state

[austinabell]

Summary of changes
Changes introduced in this pull request:

• So the issue is that the epochs elapsed value can go negative (which seems like a bug) but the u64 cast caused it to overflow and fail to send which caused a state diff
  • My guess is this is an artifact from when we were using unsigned big ints and validating, and this just didn't get noticed to remove the cast as a side effect
• Also switched the logic to more identically match Lotus for the end epoch

Fixes state diff at height 438854

Reference issue to close (if applicable)

Closes

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