sbin/hammer: Make rel_buffer() and rel_volume() check if arg is NULL

sbin/hammer: Make rel_buffer() and rel_volume() check if arg is NULL

- Some of the existing code do "if (buf) { rel_buffer(buf); }",
  but others just do "rel_buffer(buf);". Make code and things clear
  by making these rel_buffer() and rel_volume() check if the pointer
  is NULL.