Bad arguments (in C++ code) to BlindTransaction() can cause memory corruption

https://github.com/ElementsProject/elements/blob/4e023af58b697f63d0612abd712e1f8310719217/src/blind.cpp#L407-L416

If the issuance blinding key or token blinding key is specified for the input when calling BlindTransaction, but the transaction input itself have null value in `issuance.nInflationKeys`/`issuance.nAmount`, then `num_blind_attempts` will be incremented, while the size of `value_blindptrs` and `asset_blindptrs` will not be increased with another 32 bytes of random data.

(the same issue can happen with `continue` at https://github.com/ElementsProject/elements/blob/4e023af58b697f63d0612abd712e1f8310719217/src/blind.cpp#L434-L435)

If `num_blind_attempts` is larger than the size `*_blindptrs` arrays, `secp256k1_pedersen_blind_generator_blind_sum` that is called later can go over the size of the arrays, because there's no check that the sizes are equal to `num_blind_attempts + num_known_input_blinds`. This can cause crash. `secp256k1_pedersen_blind_generator_blind_sum` will also write to the last blinder, and this can cause memory corruption.

Of course this is only possible by the calling C++ code being incorrect, supplying wrong transaction data, etc. But this raises the question - are these `continue` statements actually needed ? Wouldn't it be better to replace them with returning failure ? It seems that in both cases, either the transaction structure is wrong, or the arguments ` issuance_blinding_privkey`/`token_blinding_privkey` are wrong.

Beginne in src/blind.cpp bei den Zeilen 407-416 und 434-435, verfolge anschließend die Aufrufer von BlindTransaction sowie die Konstruktion von value_blindptrs und asset_blindptrs. Überprüfe, wie ungültige Argumente für die Ausgabe oder den Blinding-Key behandelt werden, und ergänze Tests für nicht übereinstimmende Anzahlen; abgeschlossen ist die Aufgabe, wenn fehlerhafte Eingaben blind_sum nicht dazu bringen können, über die Arrays hinaus zu lesen oder zu schreiben.

Vom Indexierungsmodell aus dem Issue-Text verfasst.