Fix possible access to fromspace in MVM_repr_dimensions

Allocating the result array can cause GC which may move obj, so move the
allocation after the call to the repr function. We don't need it that early
anyway.

src/6model/reprconv.c

@@ -585,9 +585,10 @@ MVMObject * MVM_repr_dimensions(MVMThreadContext *tc, MVMObject *obj) {
     if (IS_CONCRETE(obj)) {
         MVMint64 num_dims, i;
         MVMint64 *dims;
-        MVMObject *result = MVM_repr_alloc_init(tc, tc->instance->boot_types.BOOTIntArray);
+        MVMObject *result;
         REPR(obj)->pos_funcs.dimensions(tc, STABLE(obj), obj, OBJECT_BODY(obj),
             &num_dims, &dims);
+        result = MVM_repr_alloc_init(tc, tc->instance->boot_types.BOOTIntArray);
         for (i = 0; i < num_dims; i++)
             MVM_repr_bind_pos_i(tc, result, i, dims[i]);
         return result;