Fix incorrect prune state of a signed_block issue for v2.1

[nickjjzhao]

Change Description

The variable prune_state in a signed_block should be updated after some trxs in the block get pruned that happens in prune_trxs(). Otherwise in some situation to_signed_block_v0() will try to convert a signed_block with pruned trxs to a signed_block_v0 , which is not supposed to, due to incorrect state of the variable.

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