util/ralloc: set prev-pointers correctly in ralloc_adopt

Found by inspection.

I'm not aware of any actual failures caused by this, but a precise
sequence of ralloc_adopt and ralloc_free should be able to cause
problems.

v2: make the code slightly clearer (Eric)

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/util/ralloc.c

@@ -311,10 +311,12 @@ ralloc_adopt(const void *new_ctx, void *old_ctx)
    for (child = old_info->child; child->next != NULL; child = child->next) {
       child->parent = new_info;
    }
+   child->parent = new_info;
 
    /* Connect the two lists together; parent them to new_ctx; make old_ctx empty. */
    child->next = new_info->child;
-   child->parent = new_info;
+   if (child->next)
+      child->next->prev = child;
    new_info->child = old_info->child;
    old_info->child = NULL;
 }