avconv: make -frames work for all types of streams, not just video.

[ffmpeg] / tests / ref / fate / dfa10

0, 0, 192000, 0xbabcbd55
0, 6390, 192000, 0xf00a5683
0, 12780, 192000, 0xcce90589
0, 19170, 192000, 0x8545631f
0, 25560, 192000, 0xd3ab654c
0, 31950, 192000, 0x5e0dda12
0, 38340, 192000, 0x7e94b053
0, 44730, 192000, 0x8027e68b