vp9_rdopt,handle_inter_mode: fix -Wmaybe-uninitialized warning

vp9_rdopt,handle_inter_mode: fix -Wmaybe-uninitialized warning

With gcc 13.1.1

In function ‘handle_inter_mode’,
inlined from ‘vp9_rd_pick_inter_mode_sb’ at
    ../vp9/encoder/vp9_rdopt.c:3872:17:
../vp9/encoder/vp9_rdopt.c:3142:8: warning: ‘tmp_rd’ may be used
    uninitialized [-Wmaybe-uninitialized]
 3142 |     rd = tmp_rd + RDCOST(x->rdmult, x->rddiv, rs, 0);
../vp9/encoder/vp9_rdopt.c: In function ‘vp9_rd_pick_inter_mode_sb’:
../vp9/encoder/vp9_rdopt.c:2846:15: note: ‘tmp_rd’ was declared here
 2846 |   int64_t rd, tmp_rd, best_rd = INT64_MAX;

Change-Id: I8608957cc8bbeb1ae525f3c3dad6fe9785b2a9b4