avcodec/vp9dsp: add DC only versions for idct/idct.

before:
  ./ffmpeg -v 0 -nostats -i ~/samples/vp9/etv.webm -f null -  16.29s user 0.02s system 99% cpu 16.323 total
  ./ffmpeg -v 0 -nostats -i ~/samples/vp9/etv.webm -f null -  16.32s user 0.01s system 99% cpu 16.351 total
  ./ffmpeg -v 0 -nostats -i ~/samples/vp9/etv.webm -f null -  16.27s user 0.05s system 99% cpu 16.335 total

after:
  ./ffmpeg -v 0 -nostats -i ~/samples/vp9/etv.webm -f null -  15.22s user 0.03s system 99% cpu 15.257 total
  ./ffmpeg -v 0 -nostats -i ~/samples/vp9/etv.webm -f null -  15.20s user 0.02s system 99% cpu 15.237 total
  ./ffmpeg -v 0 -nostats -i ~/samples/vp9/etv.webm -f null -  15.19s user 0.02s system 99% cpu 15.227 total
avformat/mov: Check that we have a stream before accessing it in mov_read_ares()

Fixes out of array read
Fixes: signal_sigsegv_6f1855_3910_avid_test_alpha.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/hnm4video: check offset in decode_interframe_v4() more completely

Fixes out of array reads
Fixes: signal_sigsegv_e74c1e_1092_BROCIME.HNM
Fixes: signal_sigsegv_e74e85_2620_PLAQUE0.HNM

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/hnm4video: fix write offset checks in decode_interframe_v4()

Fixes: asan_heap-oob_e76a51_1244_CASSE.HNM

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/hnm4video: fix missing newlines in error messages

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>