avcodec/speedhqenc: Call correct function

avcodec/speedhqenc: Call correct function

Up until now, the SpeedHQ encoder called a wrong function for init:
void ff_init_uni_ac_vlc(const uint8_t huff_size_ac[256],
                        uint8_t *uni_ac_vlc_len);
Yet the first argument actually used is of type RLTable; the size of
said struct is less than 256 if the size of a pointer is four, leading
to an access beyond the end of the RLTable.

This commit fixes this by calling the actually intended function:
init_uni_ac_vlc() from mpeg12enc.c. It was intended to use this
function [1], yet doing so was forgotten when the patch was actually
applied.

[1]: https://ffmpeg.org/pipermail/ffmpeg-devel/2020-July/266187.html

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>