h264: slice-mt: get last_pic_dropable from master context

Fixes fate-h264-conformance-cvnlfi2_sony_h and smllwebdl.mkv from
https://github.com/OpenELEC/OpenELEC.tv/issues/1557 .

CC: libav-stable@libav.org

libavcodec/h264.c

@@ -2601,7 +2601,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
     h->mb_mbaff        = 0;
     h->mb_aff_frame    = 0;
     last_pic_structure = s0->picture_structure;
-    last_pic_dropable  = s->dropable;
+    last_pic_dropable  = s0->dropable;
     s->dropable        = h->nal_ref_idc == 0;
     if (h->sps.frame_mbs_only_flag) {
         s->picture_structure = PICT_FRAME;