Add empty pointer check to pred buffering in rtc coding mode

This commit adds a check condition to the prediction buffering
operation used in the rtc coding mode. This resolves a unit test
warning in example/vpx_tsvc_encoder_vp9_mode_7.

Change-Id: I9fd50d5956948b73b53bd8fc5a16ee66aff61995
Merge "Add empty pointer check to pred buffering in rtc coding mode"