replace map::operator[] by .at() to avoid object creation in case of missing element.
replace map::operator[] by .at() to avoid object creation in case of missing element.

poparteu/openMVG,poparteu/openMVG,openMVG/openMVG,openMVG/openMVG,openMVG/openMVG,poparteu/openMVG,poparteu/openMVG,openMVG/openMVG,openMVG/openMVG