Normally, a target is closed when its refcount has dropped to 0, due to
not being used in any inferior anymore. I think it would make sense to
change that assert to not only check in the current inferior, but to
check in all inferiors. It would be quite bad (and a bug) to close a
target while it's still pushed in one of the non-current inferiors.
gdb/ChangeLog:
* target.c (target_close): Check in all inferiors that the
target is not pushed.