OK, I think I have an elegant solution for this.  I'm going to use
Joel's copy_command_lines function, and Joel's commands.exp tests, 
but here's what we'll do in breakpoint.c:

  1) bpstat_stop_status will make a copy of the bp commands
and put it in the bpstat (instead of merely copying the pointer).

  2) bpstat_clear, bpstat_clear_actions, and bpstat_do_actions
will free the command lines instead of simply discarding them.

Since all bpstat commands are copies instead of pointers, they
won't be affected by the deletion of the breakpoint -- and we
get to plug a memory leak as a bonus.

Here's the patch as checked in (tested on linux, no regressions):