Fix value of Reference fields when id is -1

issue3375
review1089002
(transplanted from d24d3b856c487b9904fa60923708faa751a0d6ed)

tryton/gui/window/view_form/model/field.py

@@ -779,7 +779,7 @@ class ReferenceField(CharField):
     def get(self, record):
         if (record.value.get(self.name)
                 and record.value[self.name][0]
-                and record.value[self.name][1] >= 0):
+                and record.value[self.name][1] >= -1):
             return ','.join(map(str, record.value[self.name]))
         return None