mpegenc: use avctx->slices as number of slices

[ffmpeg] / tests / ref / vsynth1 / dv

27ade3031b17214cf81c19cbf70f37d7 *./tests/data/vsynth1/dv.dv
7200000 ./tests/data/vsynth1/dv.dv
02ac7cdeab91d4d5621e7ce96dddc498 *./tests/data/dv.vsynth1.out.yuv
stddev:    6.90 PSNR: 31.34 MAXDIFF:   76 bytes:  7603200/  7603200