avconv: fix a segfault on -c copy with -filter_complex.

[ffmpeg] / tests / ref / acodec / pcm_s32be

63f0e22b4f7c5d61d75047d85f140d52 *./tests/data/acodec/pcm_s32be.mov
2118473 ./tests/data/acodec/pcm_s32be.mov
64151e4bcc2b717aa5a8454d424d6a1f *./tests/data/pcm_s32be.acodec.out.wav
stddev:    0.00 PSNR:999.99 MAXDIFF:    0 bytes:  1058400/  1058400