[elements-22.x] Backport #1134: Check the value assertion only on valid amounts

[gwillen]

(Backport of #1134)

This causes crash on elements wallet when dealing with transactions that
have explicit values and confidential assets. This creates a somewhat
serious DoS attack as the sender can cause the reciever's wallet to
crash by partially blinding the change output. To make matters worse,
the wallet initially accepts the transaction, but fails while spending
the output.

This is likely caused by a combination of two bugs:

1. The wallet's current behaviour stores the complete transaction of interest
   in CWalletTx instead of just Outpoints. Only that the spend time do we
   iterate over all outputs, try to unblind them and check which are
   isMine. When calling wtx.GetOutputValueOut() or similar calls, we hit this assertion.

While the current behaviour is okay, I think the correct way is move
the IsMine == ISMINE_NO at the start of the loop. We should not do be
any checks on outputs that are not ours. This is used in multiple
places at different parts of the codebase for different RPCs.

2. When dealing with partially blinded trasactions, ComputeBlindingData
   correctly sets value = -1, and the cache byte to 1. When getting the
   data again with GetBlindingData for explicit value and confidential
   asset, we load the precomputed data with value = -1 and assert the
   loaded value be the explicit value in the transaction. This is only true
   for explicit value and explicit asset.

The changed assertion checks that written value should be same as the
explicit value that was written only when the amounts are valid

[jhfrontz]

Compared favorably with original PR.
utack c30f9a3