Fix panic on disabled event with CallerSkipFrame

Fixes #319

event.go

@@ -698,6 +698,9 @@ func (e *Event) Interface(key string, i interface{}) *Event {
 // CallerSkipFrame instructs any future Caller calls to skip the specified number of frames.
 // This includes those added via hooks from the context.
 func (e *Event) CallerSkipFrame(skip int) *Event {
+	if e == nil {
+		return e
+	}
 	e.skipFrame += skip
 	return e
 }