Fix event.Manager category cache

Since we don't use pointer receivers, the eventCategory map was being
initialized on a copy of the manager struct.  Initialize the map in the
constructor so we properly cache the one time fetch of event categories.

event/manager.go

@@ -43,6 +43,7 @@ func NewManager(c *vim25.Client) *Manager {
 
  c: c,
 
+ eventCategory: make(map[string]string),
  eventCategoryMu: new(sync.Mutex),
  }
 
@@ -125,7 +126,7 @@ func (m Manager) eventCategoryMap(ctx context.Context) (map[string]string, error
  m.eventCategoryMu.Lock()
  defer m.eventCategoryMu.Unlock()
 
- if m.eventCategory != nil {
+ if len(m.eventCategory) != 0 {
  return m.eventCategory, nil
  }
 
@@ -137,8 +138,6 @@ func (m Manager) eventCategoryMap(ctx context.Context) (map[string]string, error
  return nil, err
  }
 
- m.eventCategory = make(map[string]string, len(o.Description.EventInfo))
-
  for _, info := range o.Description.EventInfo {
  m.eventCategory[info.Key] = info.Category
  }