vpxdec: fix use of uninitialized do_scale argument
vpxdec: fix use of uninitialized do_scale argument

This fixes a potential crash with VP8 streams that have an alt-ref on
the first frame, as well as potentially scaling the output in cases
where that wasn't desired.

Change-Id: I1fd74b5ab43329fbbffecb004dfd44de34c4dc22

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