check whether subfolder options is valid about zotfile

[brooksambrose]

Invalid subfolder names is causing a bug with JSTOR journals that are in multiple languages. If the journal name (e.g. Crime, Histoire & Sociétés / Crime, History & Societies) is used as a subfolder Zotfile will mkdir an inadvertent additional directory, then Zotero cannot find the attachment because it is looking in the wrong directory structure.

@article{10.2307/42708371,
 ISSN = {14220857, 16634837},
 URL = {http://www.jstor.org/stable/42708371},
 author = {Philippe Chassaigne and W. Heppel},
 journal = {Crime, Histoire & Sociétés / Crime, History & Societies},
 number = {2},
 pages = {23--55},
 publisher = {Librairie Droz},
 title = {Popular representations of crime: the crime broadside — a subculture of violence in Victorian Britain?},
 volume = {3},
 year = {1999}
}