Fix invalid iterator access in TimeInterpolatableBuffer

[virtuald]

Found using a fuzzer. I ran this fix for several hundred thousand executions without any errors.

[virtuald]

In case anyone cares, here's the fuzz driver I wrote:

static constexpr units::second_t kBufferDuration = 1.5_s;

int main(int argc, char **argv) {

    frc::TimeInterpolatableBuffer<int> buffer(kBufferDuration, [](const int& start, const int &end, double t) {
        return 4;
    });

    std::ifstream t(argv[1]);
    std::stringstream fb;
    fb << t.rdbuf();
    auto str = fb.str();
    auto *buf = str.c_str();
    auto end = str.size();

    int i = 0;
    double ts;

    while (i < str.size()) {
        auto c = buf[i];
        i += 1;
        switch (((int)c) % 2) {
        case 0:
            // sample some timestamp (needs time)
            if (i + 8 >= end) {
                return 0;
            }

            ts = *(double*)(buf + i);
            buffer.Sample(units::time::second_t(ts));
            i += 8;
            break;
        case 1:
            // add some sample (needs time, sample)
            if (i + 8 >= end) {
                return 0;
            }

            ts = *(double*)(buf + i);
            buffer.AddSample(units::time::second_t(ts), 0);

            i += 8;
            break;
        }
    }
}