Fixes a bug that caused the CLI wallet to be unable to decrypt memos in some transactions sent by the web wallet, and vice versa. The shared secret used in to encrypt the memos is derived by computing a 256-bit quantity from the public and private keys.  When the high- order byte of this quantity is zero, this implementation was omitting the high-order byte, producing a 248-bit number instead.  When passed through the hash function to create the symmetric key, this generated a different key from the C++ implementation which always generates 256-bits. This fix will restore compatibility with the CLI wallet, but will prevent the web wallet from decoding any incompatible memos that were have already been generated.
Fixes a bug that caused the CLI wallet to be unable to decrypt memos
in some transactions sent by the web wallet, and vice versa.
The shared secret used in to encrypt the memos is derived by computing
a 256-bit quantity from the public and private keys.  When the high-
order byte of this quantity is zero, this implementation was omitting
the high-order byte, producing a 248-bit number instead.  When passed
through the hash function to create the symmetric key, this generated
a different key from the C++ implementation which always generates
256-bits.
This fix will restore compatibility with the CLI wallet, but will
prevent the web wallet from decoding any incompatible memos that were
have already been generated.

BitSharesEurope/testnet.bitshares.eu,BitSharesEurope/graphene-ui-testnet,BitSharesEurope/graphene-ui-testnet,poqdavid/graphene-ui,BitSharesEurope/graphene-ui-testnet,bitshares/bitshares-ui,bitshares/bitshares-ui,cryptonomex/graphene-ui,BitSharesEurope/wallet.bitshares.eu,BitSharesEurope/testnet.bitshares.eu,poqdavid/graphene-ui,openledger/graphene-ui,BunkerChainLabsInc/freedomledger-wallet,BitSharesEurope/testnet.bitshares.eu,BitSharesEurope/wallet.bitshares.eu,trendever/bitshares-ui,BitSharesEurope/testnet.bitshares.eu,BitSharesEurope/wallet.bitshares.eu,bitshares/bitshares-2-ui,trendever/bitshares-ui,BunkerChainLabsInc/freedomledger-wallet,trendever/bitshares-ui,openledger/graphene-ui,BitSharesEurope/graphene-ui-testnet,bitshares/bitshares-2-ui,trendever/bitshares-ui,bitshares/bitshares-2-ui,bitshares/bitshares-ui,bitshares/bitshares-ui,cryptonomex/graphene-ui,poqdavid/graphene-ui,BitSharesEurope/wallet.bitshares.eu,BunkerChainLabsInc/freedomledger-wallet,poqdavid/graphene-ui,openledger/graphene-ui,BunkerChainLabsInc/freedomledger-wallet,cryptonomex/graphene-ui