Since the av_malloc() may fail and return NULL pointer,
it is needed that the 's->edge_emu_buffer' should be checked
whether the new allocation is success.

Fixes: d1472386 ("VP3: fix decoding of videos with stride > 2048")
Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>