h_subpel_filters_inner[i] and h_subpel_filters_outer[i / 2]
belong together and the former allows the range 0..6,
so the latter needs to support 0..3. But it has only three
elements. Add another one.
The value for the last element has been guesstimated
from subpel_filters in libavcodec/vp8dsp.c.

This is also intended to fix FATE-failures with UBSan here:
https://fate.ffmpeg.org/report.cgi?time=20240312011016&slot=ppc-linux-gcc-13.2-ubsan-altivec-qemuTested-by: Sean McGovern <gseanmcg@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>