Fix use after free introduced by $_hit_bpnum/$_hit_locno variables.

If the commands of the bpstat bs contain commands such as step or next or
continue, the BS and its commands are freed by execute_control_command.

So, we cannot remember the BS that was printed. Instead, remember
the bpnum and locno.

Regtested on debian/amd64 and re-run a few tests under valgrind.