Handles duplicated NAL start code to fix crash on HLS streams.

Some youtube live streams are encoded having duplicated NAL start code,
for instance, 00 00 01 00 00 00 01 ....

In previous code, zero NAL size causes crash by CHECK_GT(nalsize, 0)
macro. With this patch, duplicated NAL start code will be just ignored.

TESTED=Played problematic Youtube Live streams.

Change-Id: I1d76f111a34bd29cb09b037eb1b0626fe5f5b140