sealing pipeline: Fix panic on padding pieces in WaitDeals

[magik6k]

Related Issues

Fixes #11706

Proposed Changes

• Don't use padding pieces (which have no deal info) for calculating deal end epoch in WaitDeals
• Make the pieceinfo methods safer
• Catch panics in the sealing pipeline

Additional Info

Checklist

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[LexLuthr]

Shouldn't we also handle?

func (sp *SafeSectorPiece) Key() piece.PieceKey {
	return sp.real.DealInfo.Key()
}
func (sp *SafeSectorPiece) Valid(nv network.Version) error {
	return sp.real.DealInfo.Valid(nv)
}