tip_build_inter_predictors_8x8: zero-out mbmi_buf

This avoids the following valgrind warning seen in some cases:

```
==244734== Conditional jump or move depends on uninitialised value(s)
==244734==    at 0x67355B: is_global_mv_block (blockd.h:1079)
==244734==    by 0x6790A8: av1_opfl_build_inter_predictor
(reconinter.c:1194)
==244734==    by 0x6A768D: tip_build_inter_predictors_8x8 (tip.c:908)
```

Note: All other 'mbmi' buffers are already zeroed out using either `av1_zero` or `calloc`; this was the only instance missing zero-ing out logic.

No change in stats observed.

av1/common/tip.c

diff --git a/av1/common/tip.c b/av1/common/tip.c
index f8c6a69..2c11671 100644
--- a/av1/common/tip.c
+++ b/av1/common/tip.c
@@ -1012,6 +1012,7 @@
   const int comp_bh = bh >> ss_y;
 
   MB_MODE_INFO mbmi_buf;
+  av1_zero(mbmi_buf);
   MB_MODE_INFO *mbmi = &mbmi_buf;
 
   int_mv mv_refined[2 * 4];