BUG: Fix crash on saving node named with a single number
requested to merge github/fork/naucoin/3991-Node-names-with-number-colon-confuse-save-dialog-lead-to-crash into master
A previous fix[1] to avoid crashes on windows when saving volumes named "1:xxxx" leads to a crash when a node name is just a single number (the first part of the check passes, but crashes when trying to get the second character in the name string). This fix adds a string length check so that valid node names like "1" or "2" will not cause a crash on save.
[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=24294
Issue #3991