webm_info: check vp9 ParseUncompressedHeader return
webm_info: check vp9 ParseUncompressedHeader return

avoids potential side effects

BUG=webm:1416,webm:1417

Change-Id: I4acd4c9f86be20fa3c06af6c87c1ce538a50c84c

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm