svq1enc: Set picture_structure correctly

This fixes assert failures when running in debug mode.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/svq1enc.c

@@ -486,6 +486,7 @@ static av_cold int svq1_encode_init(AVCodecContext *avctx)
 
     s->avctx= avctx;
     s->m.avctx= avctx;
+    s->m.picture_structure = PICT_FRAME;
     s->m.me.temp      =
     s->m.me.scratchpad= av_mallocz((avctx->width+64)*2*16*2*sizeof(uint8_t));
     s->m.me.map       = av_mallocz(ME_MAP_SIZE*sizeof(uint32_t));