[Math] Fix gcc warning in recently-introduced TMath::ModeHalfSample

[guitargeek]

Follows up on d0e79a5.

Fixes the following compiler warning that relates to doing data() + 1 which is not valid if the vector would be empty:

In member function ‘void std::__new_allocator<_Tp>::deallocate(_Tp*, size_type) [with _Tp = double]’,
    inlined from ‘static void std::allocator_traits<std::allocator<_CharT> >::deallocate(allocator_type&, pointer, size_type) [with _Tp = double]’ at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/alloc_traits.h:550:23,
    inlined from ‘void std::_Vector_base<_Tp, _Alloc>::_M_deallocate(pointer, std::size_t) [with _Tp = double; _Alloc = std::allocator<double>]’ at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:392:19,
    inlined from ‘void std::_Vector_base<_Tp, _Alloc>::_M_deallocate(pointer, std::size_t) [with _Tp = double; _Alloc = std::allocator<double>]’ at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:388:7,
    inlined from ‘std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = double; _Alloc = std::allocator<double>]’ at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:371:15,
    inlined from ‘std::vector<_Tp, _Alloc>::~vector() [with _Tp = double; _Alloc = std::allocator<double>]’ at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:751:7,
    inlined from ‘Double_t TMath::ModeHalfSample(Long64_t, const T*, const Double_t*) [with T = short int]’ at /home/rembserj/code/root/root_build/include/TMath.h:1540:1:
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/new_allocator.h:172:26: warning: ‘void operator delete(void*, std::size_t)’ called on a pointer to an unallocated object ‘18446744073709551608’ [-Wfree-nonheap-object]

[github-actions]

Test Results

    21 files      21 suites   3d 15h 6m 34s ⏱️
 3 615 tests  3 474 ✅  0 💤 141 ❌
74 160 runs  74 002 ✅ 17 💤 141 ❌

For more details on these failures, see this check.

Results for commit bd97cbd.

♻️ This comment has been updated with latest results.

[guitargeek]

Hi @ferdymercury! Sorry about the original version of the PR, I understood now that the logic was as intended, and your comments then went more in the direction of code optimization, which was not my original intention with this PR.

So I went back to simply fixing the warning.

[ferdymercury]

Thanks!

Could you also remove "are unique " from the sentence in line 1503?
Or change it to "are unique (if w != nullptr)"

[guitargeek]

Actually I'm not sure if std::next(values.begin()) is better than data() + 1, even though there is no warning anymore 🙂

I'll close this PR for now, since the warning is not in the CI, and I'm also not affected by it personally anymore, since I now use clang instead of gcc for faster compilation locally. And with clang 19, I don't get this warning.