VT: fixes possible nullptr case in ARTPConnection.

[Problem] null pointer dereference happened in onPollStream()
[Cause] there is a possible case during getSelfID() called
  if mSource(vector) doesn't have any node.
[Solution] getSelfID() should be called by valid pointer.

Bug: 175657734

Change-Id: Ibec81df1c0455cefbd0414b7475fc622e5f4ddce
Signed-off-by: Byeongjo Park <bjo.park@samsung.com>