mem: add mem_destructor

This way a destruct handler can assigned after mem_alloc.

include/re_mem.h

@@ -26,6 +26,7 @@ void    *mem_zalloc(size_t size, mem_destroy_h *dh);
 void    *mem_realloc(void *data, size_t size);
 void    *mem_reallocarray(void *ptr, size_t nmemb,
 			  size_t membsize, mem_destroy_h *dh);
+void 	mem_destructor(void *data, mem_destroy_h *dh);
 void    *mem_ref(void *data);
 void    *mem_deref(void *data);
 uint32_t mem_nrefs(const void *data);

src/mem/mem.c

@@ -262,6 +262,27 @@ void *mem_reallocarray(void *ptr, size_t nmemb, size_t membsize,
 }
 
 
+/**
+ * Set or unset a destructor for a memory object
+ *
+ * @param data Memory object
+ * @param dh   called when destroyed, NULL for remove
+ */
+void mem_destructor(void *data, mem_destroy_h *dh)
+{
+	struct mem *m;
+
+	if (!data)
+		return;
+
+	m = ((struct mem *)data) - 1;
+
+	MAGIC_CHECK(m);
+
+	m->dh = dh;
+}
+
+
 /**
  * Reference a reference-counted memory object
  *