Fix a warning on Clang
Fix a warning on Clang

The warning text is: moving a local object in a return statement prevents copy
elision [-Wpessimizing-move]

chemfiles/chemfiles,lscalfi/chemfiles,chemfiles/chemfiles,chemfiles/chemfiles,lscalfi/chemfiles,Luthaf/Chemharp,Luthaf/Chemharp,Luthaf/Chemharp,lscalfi/chemfiles,lscalfi/chemfiles,chemfiles/chemfiles