MultiViewParams::findTileNearestCams erroneously assumes non-empty array, leading to illegal read when indexing first element

[SFN-eu]

While fixing #2093, I discovered that MultiViewParams::findTileNearestCams assumes that at least one tCamera id has a positive score, than calls qsort on an array it assumes to be non-empty.

The offending code is as follows: (MultiViewParams.cpp 650:659)

    std::vector<SortedId> ids;
    ids.reserve(tcScore.size());

    for (const auto& tcScorePair : tcScore)
    {
        if (tcScorePair.second > 0.0f)
            ids.push_back(SortedId(tcScorePair.first, tcScorePair.second));
    }

    qsort(&ids[0], ids.size(), sizeof(SortedId), qsortCompareSortedIdDesc);

    // ensure the ideal number of target cameras is not superior to the actual number of cameras
    const int maxTc = std::min(std::min(getNbCameras(), nbNearestCams), static_cast<int>(ids.size()));
    out.reserve(maxTc);

    for (int i = 0; i < maxTc; ++i)
        out.push_back(ids[i].id);

    return out;

(It even causes a SIGABRT when compiled in debug mode, because gcc bound checks the subscript operator when debugging)

A simple if statement could fix this and just return out before the sort if ids is empty. Again, like with #2093, will try to push up a patch and open a PR.