mov_read_ctts ignores ctts entries having count <= 0. Generally, the
aggregate of all ctts entries' count fields resulting from mov_read_ctts
can be less than the corresponding sample_count.

mov_build_index attempts to normalize any existing ctts_data counts to
be 1, to make a 1-1 mapping of a ctts_data entry to a sample.

That 1-1 mapping left a tail of uninitialized ctts_data entries when the
aggregate, normalized ctts_count < sample_count.

Even more generally, later usage of ctts_data may depend on the entire
ctts_allocated_size having been initialized.

This change memsets the entire allocation of the normalized ctts_data in
mov_build_index, to prevent use of uninitialized data later.

BUG=816787

Change-Id: I7fd7db255e3aeed076ee32c90cb2df211741c052
Reviewed-on: https://chromium-review.googlesource.com/947110Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>