fix PGRange to Block.Range conversion

apps/explorer/lib/explorer/chain/block/range.ex

@@ -136,10 +136,12 @@ defmodule Explorer.Chain.Block.Range do
 
   def load(_), do: :error
 
+  defp parse_upper(%PGRange{upper: :unbound}), do: :infinity
   defp parse_upper(%PGRange{upper: nil}), do: :infinity
   defp parse_upper(%PGRange{upper: upper, upper_inclusive: true}), do: upper
   defp parse_upper(%PGRange{upper: upper, upper_inclusive: false}), do: upper - 1
 
+  defp parse_upper(%PGRange{lower: :unbound}), do: :negative_infinity
   defp parse_lower(%PGRange{lower: nil}), do: :negative_infinity
   defp parse_lower(%PGRange{lower: lower, lower_inclusive: true}), do: lower
   defp parse_lower(%PGRange{lower: lower, lower_inclusive: false}), do: lower + 1