avcodec/vc1dec: Postpone allocating sprite frame to avoid segfault

avcodec/vc1dec: Postpone allocating sprite frame to avoid segfault

Up until now, the VC-1 decoders allocated an AVFrame for usage with
sprites during vc1_decode_init(); yet said AVFrame can be freed if
(re)initializing the context (which happens ordinarily during decoding)
fails. The AVFrame does not get allocated again lateron in this case,
leading to segfaults.

Fix this by moving the allocation of said frame immediately before it is
used (this also means that said frame won't be allocated at all any more
in case of a regular (i.e. non-image) stream).

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