fundamental: Don't trace uninitialized memory

This was tracing both fundamental instances and prototypes. Only the
prototype holds a reference to the constructor name which needs to be
traced; in the instance case, we were just tracing random memory.

Since the same check is made several different ways already in the file,
we factor it out into a separate fundamental_is_prototype() function.

https://bugzilla.gnome.org/show_bug.cgi?id=776966