[FIX] stock_account: valuation entry for reconciled neg quant not always possible

Skip the creation of the corrective valuation entry
when a negative quant is reconciled with an incoming
shipment, when:
 - the cost has not changed, so the journal entry would
   be useless (credit/debit = 0)
 - or, when the accounting period for the move causing
   the negative quant is already closed, and must not be
   updated (presumably the valuation was manually
   set before closing that period)

addons/stock_account/stock_account.py

@@ -79,13 +79,22 @@ class stock_quant(osv.osv):
         '''
         if context is None:
             context = {}
+        account_period = self.pool['account.period']
         super(stock_quant, self)._price_update(cr, uid, quant_ids, newprice, context=context)
-        ctx = context.copy()
         for quant in self.browse(cr, uid, quant_ids, context=context):
             move = self._get_latest_move(cr, uid, quant, context=context)
-            # this is where we post accounting entries for adjustment
-            ctx['force_valuation_amount'] = newprice - quant.cost
-            self._account_entry_move(cr, uid, [quant], move, context=ctx)
+            valuation_update = newprice - quant.cost
+            # this is where we post accounting entries for adjustment, if needed
+            if not quant.company_id.currency_id.is_zero(valuation_update):
+                # adjustment journal entry needed, cost has been updated
+                period_id = (context.get('force_period') or
+                                 account_period.find(cr, uid, move.date, context=context)[0])
+                period = account_period.browse(cr, uid, period_id, context=context)
+                # If neg quant period already closed (likely with manual valuation), skip update
+                if period.state != 'done':
+                    ctx = dict(context, force_valuation_amount=valuation_update)
+                    self._account_entry_move(cr, uid, [quant], move, context=ctx)
+
             #update the standard price of the product, only if we would have done it if we'd have had enough stock at first, which means
             #1) the product cost's method is 'real'
             #2) we just fixed a negative quant caused by an outgoing shipment