Avoid memory corruption when registering boxed synonyms repeatedly

https://bugzilla.gnome.org/show_bug.cgi?id=704143
Avoid misusing PL_na

We used to do "newSVpv (string, PL_na)" when we didn't know the string's
length.  That's not correct, and breaks if other XS modules write to PL_na,
which is what PL_na is intended for.  Simply use "newSVpv (string, 0)" instead.
Fix a compiler warning

https://bugzilla.gnome.org/show_bug.cgi?id=700275
Updated copyright dates
Stable release 1.302