Add VSB_destroy(). It's like VSB_delete() but it NULL's the pointer.

include/vsb.h

@@ -75,6 +75,7 @@ int		 VSB_finish(struct vsb *);
 char		*VSB_data(const struct vsb *);
 ssize_t		 VSB_len(const struct vsb *);
 void		 VSB_delete(struct vsb *);
+void		 VSB_destroy(struct vsb **);
 #define VSB_QUOTE_NONL	1
 void		 VSB_quote(struct vsb *s, const char *p, int len, int how);
 void		 VSB_indent(struct vsb *, int);

lib/libvarnish/vsb.c

@@ -489,6 +489,13 @@ VSB_delete(struct vsb *s)
 		SBFREE(s);
 }
 
+void
+VSB_destroy(struct vsb **s)
+{
+	VSB_delete(*s);
+	*s = NULL;
+}
+
 /*
  * Quote a string
  */