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.

py/gc.c

@@ -434,12 +434,13 @@ void *gc_alloc(size_t n_bytes, bool has_finaliser) {
         ATB_FREE_TO_TAIL(bl);
     }
 
-    GC_EXIT();
-
     // get pointer to first block
+    // we must create this pointer before unlocking the GC so a collection can find it
     void *ret_ptr = (void*)(MP_STATE_MEM(gc_pool_start) + start_block * BYTES_PER_BLOCK);
     DEBUG_printf("gc_alloc(%p)\n", ret_ptr);
 
+    GC_EXIT();
+
     // zero out the additional bytes of the newly allocated blocks
     // This is needed because the blocks may have previously held pointers
     // to the heap and will not be set to something else if the caller