get_full_history: division by zero

On git master 553993b4d01605d5d745e142c64561c31c342051.

Traceback (most recent call last):
  File "...\electrum\gui\qt\__init__.py", line 217, in start_new_window
    w = self.create_window_for_wallet(wallet)
  File "...\electrum\gui\qt\__init__.py", line 179, in create_window_for_wallet
    w = ElectrumWindow(self, wallet)
  File "...\electrum\gui\qt\main_window.py", line 199, in __init__
    self.load_wallet(wallet)
  File "...\electrum\lib\util.py", line 288, in <lambda>
    return lambda *args, **kw_args: do_profile(func, args, kw_args)
  File "...\electrum\lib\util.py", line 284, in do_profile
    o = func(*args, **kw_args)
  File "...\electrum\gui\qt\main_window.py", line 342, in load_wallet
    self.history_list.update()
  File "...\electrum\gui\qt\util.py", line 499, in update
    self.on_update()
  File "...\electrum\lib\util.py", line 288, in <lambda>
    return lambda *args, **kw_args: do_profile(func, args, kw_args)
  File "...\electrum\lib\util.py", line 284, in do_profile
    o = func(*args, **kw_args)
  File "...\electrum\gui\qt\history_list.py", line 215, in on_update
    r = self.wallet.get_full_history(domain=self.get_domain(), from_timestamp=self.start_timestamp, to_timestamp=self.end_timestamp, fx=fx)
  File "...\electrum\lib\util.py", line 288, in <lambda>
    return lambda *args, **kw_args: do_profile(func, args, kw_args)
  File "...\electrum\lib\util.py", line 284, in do_profile
    o = func(*args, **kw_args)
  File "...\electrum\lib\wallet.py", line 1043, in get_full_history
    acquisition_price = - value / Decimal(COIN) * self.average_price(tx_hash, fx.timestamp_rate, fx.ccy)
  File "...\electrum\lib\wallet.py", line 1753, in average_price
    return total_price / (input_value/Decimal(COIN))
decimal.InvalidOperation: [<class 'decimal.DivisionUndefined'>]

Start in lib/wallet.py at get_full_history and average_price, using the traceback and the GUI history-loading path as the entry point. Reproduce the failure while opening the wallet history, then inspect the zero-value calculation and add coverage for the affected case. Done means history loads without decimal.InvalidOperation and the regression is tested.