Don't require prevtx for coins with BIP-143 fork id

[jhoenicke]

Is using coins.by_name okay here, to check for force_bip143?

This is necessary to get electron-cash working again (plus the obvious update of the plugin in electroncash – I'm working on that).

[matejcik]

it's ok, but please modify it so that it doesn't crash if the coin_name is unknown.

something like:

if coin_name in coins.by_name:
    load_prevtxes = coins.by_name[coin_name]["force_bip143"]
else:
    load_prevtxes = False

also note that you could work around this by providing prev_txes=defaultdict(TransactionType) on the one hand i'm sort of reluctant to recommend it :)) on the other, i'm considering adding this as a constant to btc.py, perhaps IGNORE_PREV_TXES as a special value for this purpose?

[jhoenicke]

I added the if coin_name in coins.by_name you suggested, just fixed the polarity (load_prevtxes is true if force_bip143 is false).

I also added the defaultdict workaround to electroncash so that it works with trezor 0.11.1.

[matejcik]

merged, thanks!