vc1dec: add flush function for WMV9 and VC-1 decoders

CC: libav-stable@libav.org
(cherry picked from commit 4dc8c8386e)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/vc1dec.c

@@ -5812,6 +5812,7 @@ AVCodec ff_vc1_decoder = {
     .init           = vc1_decode_init,
     .close          = vc1_decode_end,
     .decode         = vc1_decode_frame,
+    .flush          = ff_mpeg_flush,
     .capabilities   = CODEC_CAP_DR1 | CODEC_CAP_DELAY,
     .long_name      = NULL_IF_CONFIG_SMALL("SMPTE VC-1"),
     .pix_fmts       = ff_hwaccel_pixfmt_list_420,
@@ -5827,6 +5828,7 @@ AVCodec ff_wmv3_decoder = {
     .init           = vc1_decode_init,
     .close          = vc1_decode_end,
     .decode         = vc1_decode_frame,
+    .flush          = ff_mpeg_flush,
     .capabilities   = CODEC_CAP_DR1 | CODEC_CAP_DELAY,
     .long_name      = NULL_IF_CONFIG_SMALL("Windows Media Video 9"),
     .pix_fmts       = ff_hwaccel_pixfmt_list_420,