py/gc: Fix GC+thread bug where ptr gets lost because it's not computed.

GC_EXIT() can cause a pending thread (waiting on the mutex) to be
scheduled right away.  This other thread may trigger a garbage
collection.  If the pointer to the newly-allocated block (allocated by
the original thread) is not computed before the switch (so it's just left
as a block number) then the block will be wrongly reclaimed.

This patch makes sure the pointer is computed before allowing any thread
switch to occur.