avcodec/motion_est_template: Avoid using last + 1 element of array

For an int array[8][2] using &array[8][0] (which is an int*
pointing to the element beyond the last element of array)
triggers a "runtime error: index 8 out of bounds for type 'int[8][2]'"
from (Clang-)UBSan in the fate-vsynth(1|2|_lena)-snow tests.

I don't know whether this is really undefined behaviour or does not
actually fall under the "pointer arithmetic with the element beyond
the last element of the array is allowed as long as it is not
accessed" exception". All I know is that the code itself does not
read from beyond the last element of the array.

Nevertheless rewrite the code to a form that UBSan does not complain
about.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/motion_est_template.c

@@ -281,7 +281,7 @@ static int qpel_motion_search(MpegEncContext * s,
                     for(i=0; i<8; i++){
                         if(score < best[i]){
                             memmove(&best[i+1], &best[i], sizeof(int)*(7-i));
-                            memmove(&best_pos[i+1][0], &best_pos[i][0], sizeof(int)*2*(7-i));
+                            memmove(&best_pos[i + 1], &best_pos[i], sizeof(*best_pos) * (7 - i));
                             best[i]= score;
                             best_pos[i][0]= nx + 4*mx;
                             best_pos[i][1]= ny + 4*my;