Since we don't memset the unicode allocations any more, and
we now use a precise gc handler, we have to be careful that
unicode objects are valid enough whenever a collection could
happen.

The issue was that "unicode->str = gc_alloc(...)" could cause
a collection, in which case the collector would go and see a
bogus value in the ->str field.  Now, do the str allocation
first (since it is UNTRACKED) and then do the precise allocation
second.