Fix two bugs in slice-min-mbs and slices-max

Slices-max broke slice-max-size when slice-max wasn't used.
Slice-min-mbs broke in rare cases near the end of a threadslice.
x86: AVX2 deblock strength

30->18 cycles
x86: AVX2 high bit-depth intra_sad_x3_8x8

43->26 cycles
x86: AVX2 intra_sad_x3_8x8c

30->23 cycles