Allow an amount to be zero, even if cost is non-zero

The only real change is that exchanges of zero-amount will not result in
any exchange at all, just the "motions".

Fixes 6DAB9FE3-1399-4727-9AB1-FE413FB85A03

src/commodity.cc

@@ -418,11 +418,13 @@ commodity_t::exchange(const amount_t&		  amount,
   if (commodity.annotated)
     current_annotation = &as_annotated_commodity(commodity).details;
 
-  amount_t per_unit_cost = (is_per_unit ? cost : cost / amount).abs();
+  amount_t per_unit_cost =
+    (is_per_unit || amount.is_realzero() ? cost : cost / amount).abs();
 
   DEBUG("commodity.prices.add", "exchange: per-unit-cost = " << per_unit_cost);
 
-  exchange(commodity, per_unit_cost, moment ? *moment : CURRENT_TIME());
+  if (! per_unit_cost.is_realzero())
+    exchange(commodity, per_unit_cost, moment ? *moment : CURRENT_TIME());
 
   cost_breakdown_t breakdown;
   breakdown.final_cost = ! is_per_unit ? cost : cost * amount;

test/regress/6DAB9FE3.test

@@ -0,0 +1,10 @@
+reg -E
+<<<
+2009/01/01 Sample
+   assets  0 FOO @ $8.88
+   equity
+>>>1
+09-Jan-01 Sample                assets                            0            0
+                                equity                            0            0
+>>>2
+=== 0