lavf/utils.c Protect against accessing entries[nb_entries]

In ff_index_search_timestamp(), if b == num_entries,
m == num_entries - 1, and entries[m].flags & AVINDEX_DISCARD_FRAME is
true, then the search for the next non-discarded packet could access
entries[nb_entries], exceeding its bounds. This change adds a protection
against that scenario. Reference: https://crbug.com/666770

Reviewed-by: Sasi Inguva <isasi@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
