avcodec/vp9: Fix stack-buffer overflow with VP9 VDPAU available

avcodec/vp9: Fix stack-buffer overflow with VP9 VDPAU available

ccca62ef991f0a47dfa30c3e822d91294b8afe4c added new VP9 VDPAU profiles
and as a consequence AV_PIX_FMT_VDPAU can now be twice in the list of
pixel formats used for format negotiation by ff_thread_get_format(); yet
there is only one entry in said list reserved for VDPAU, leading to a
stack-buffer overflow. This commit fixes this by making sure that
AV_PIX_FMT_VDPAU will not occur twice in said list.

Fixes Coverity ticket 1468046.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>