avcodec/ppc/vp8dsp_altivec: Fix out-of-bounds access

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>

libavcodec/ppc/vp8dsp_altivec.c

@@ -50,11 +50,12 @@ static const vec_s8 h_subpel_filters_inner[7] =
 // for 6tap filters, these are the outer two taps
 // The zeros mask off pixels 4-7 when filtering 0-3
 // and vice-versa
-static const vec_s8 h_subpel_filters_outer[3] =
+static const vec_s8 h_subpel_filters_outer[4] =
 {
     REPT4(0, 0, 2, 1),
     REPT4(0, 0, 3, 3),
     REPT4(0, 0, 1, 2),
+    REPT4(0, 0, 0, 0),
 };
 
 #define LOAD_H_SUBPEL_FILTER(i) \