Fix Xing seeking when TOC[0] == 0

A file purchased from the Amazon mp3 store had two 0 entries at the
start of the TOC (presumably because there was a lot of silence at
the beginning, which compresses well), which made TOC based seeking
think the TOC was not valid.
b/6170518

Change-Id: Ie7135b4af06ebb925f63a374be780cf3a9404347