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.