free shape storage last (#6511)

Some decref-triggered runtime methods need the shape

Fixes #6509

Co-authored-by: Steven Johnson <srj@google.com>
(cherry picked from commit 5c33902)

src/runtime/HalideBuffer.h

@@ -991,8 +991,8 @@ class Buffer {
      * this is the last reference to it. Will assert fail if there are
      * weak references to this Buffer outstanding. */
     ~Buffer() {
-        free_shape_storage();
         decref();
+        free_shape_storage();
     }
 
     /** Get a pointer to the raw halide_buffer_t this wraps. */