Segfault within coordgen for molecule

[greglandrum]

This one was reported by @lucasmorin222 in the RDKit tracker (rdkit/rdkit#4845), but I've confirmed that it's a coordgen problem.

I don't have a straight reproducible for coordgen, but this bit of C++ using RDKit+Coordgen results in a segfault:

    auto m1 = "C1C=CC=C2C1=CSC=C3C(=O)OOOC(=O)C(=CC(=O)N3)OC2=O"_smiles;
    CoordGen::addCoords(*m1);

The crash occurs here:
https://github.com/schrodinger/coordgenlibs/blob/master/sketcherMinimizerAtom.cpp#L377
because lastPoppedIndex is 1 and neighs.size() is 1.

I'll do a PR.