Fix bug in InputGenerator::generateCoordinateBlock()
Fix bug in InputGenerator::generateCoordinateBlock()

This fixes a bug in which the generateCoordinateBlock() method would
attempt to resize an empty string to numeric_limits<size_t>::max()
which would throw std::length_error. The code now checks that the
string is not empty before attempting to remove the last character.

Change-Id: I90a60297a14b436b5ca759343a5d6e25ce89b844

ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,cjh1/mongochemweb-avogadrolibs,cjh1/mongochemweb-avogadrolibs,ghutchis/avogadrolibs