avcodec/x86/lossless_videoencdsp: Fix handling of small widths

Fixes out of array access
Fixes: crash-huf.avi

Regression since: 6b41b4414934cc930468ccd5db598dd6ef643987

This could also be fixed by adding checks in the C code that calls the dsp

Found-by: Zhibin Hu and 连一汉 <lianyihan@360.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
