HistFactory: invalid Sample names cause segfault

When running hist2workspace, providing an invalid Sample name (a name which begins with a number) causes a segfault when generating Asimov data. It's not at all obvious to the user that the invalid Sample name is causing this problem.

A minimal setup to replicate the crash is to run the example.xml setup included in $ROOTSYS/tutorial/histfactory, after changing the Sample names in config/example_channel.xml to something like 1signal, 2background, 3background then running hist2workspace config/example.xml.