CVE-2021-29601: Integer overflow in TFLite concatentation

The TFLite implementation of concatenation is vulnerable to an integer overflow issue:

for (int d = 0; d < t0->dims->size; ++d) {
if (d == axis) {
sum_axis += t->dims->data[axis];
} else {
TF_LITE_ENSURE_EQ(context, t->dims->data[d], t0->dims->data[d]);
}
}

An attacker can craft a model such that the dimensions of one of the concatenation input overflow the values of int. TFLite uses int to represent tensor dimensions, whereas TF uses int64. Hence, valid TF models can trigger an integer overflow when converted to TFLite format.