Fixes endless loop bug when entering an invalid year (via keyboard) when...

[jonathonadler]

... format is dd/mm/yyyy

[eternicode]

Actually, the deeper problem is that we're getting NaN values for some fields (for example, when 'may' is given for a 'dd' chunk); this turns the date into an Invalid Date, which then fubars the m parser with the loop.

#225 fixes this deeper down, for all parsers. Thanks for this try, though!