BUG: Fix continuing to buy 0 units resulting in AssertionError

Fixes #179

Thanks @sdmovie

Author: kernc

backtesting/backtesting.py

@@ -879,6 +879,8 @@ def _process_orders(self):
                         # so it will only be closed partially
                         self._reduce_trade(trade, price, need_size, time_index)
                         need_size = 0
+
+                    if not need_size:
                         break
 
             # If we don't have enough liquidity to cover for the order, cancel it

backtesting/test/_test.py

@@ -469,6 +469,16 @@ def coroutine(self):
 
         self._Backtest(coroutine).run()
 
+    def test_close_trade_leaves_needsize_0(self):
+        def coroutine(self):
+            self.buy(size=1)
+            self.buy(size=1)
+            yield
+            if self.position:
+                self.sell(size=1)
+
+        self._Backtest(coroutine).run()
+
 
 class TestOptimize(TestCase):
     def test_optimize(self):