h264: fix AVDISCARD_NONKEY for some interlaced content

When skip_frame is set to _NONKEY the decoder skips everything except intra
slices, which breaks frames that consist of an intra field together with any
other field type; half the frame becomes garbage. This patch fixes the issue by
letting non-intra slices through if they're part of a keyframe.

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

libavcodec/h264.c

@@ -1507,7 +1507,7 @@ again:
                     (avctx->skip_frame < AVDISCARD_BIDIR  ||
                      sl->slice_type_nos != AV_PICTURE_TYPE_B) &&
                     (avctx->skip_frame < AVDISCARD_NONKEY ||
-                     sl->slice_type_nos == AV_PICTURE_TYPE_I) &&
+                     h->cur_pic_ptr->f->key_frame) &&
                     avctx->skip_frame < AVDISCARD_ALL) {
                     if (avctx->hwaccel) {
                         ret = avctx->hwaccel->decode_slice(avctx,