gdb/remote: fix use after free bug

gdb/remote: fix use after free bug

This commit:

  commit 288712bbaca36bff6578bc839ebcdc3707662f81
  Date:   Mon Nov 22 15:16:27 2021 +0000

      gdb/remote: use scoped_restore to control starting_up flag

introduced a use after free bug.  The scoped restore added in the
above commit resets a flag within a remote_target's remote_state
object.

However, in some situations, the remote_target can be unpushed before
the error is thrown.  If the only reference to the target is the one
in the target stack, then unpushing the target will cause the
remote_target to be deleted, which, in turn, will delete the
remote_state object.  The scoped restore will then try to reset the
flag within a deleted object.

This problem was caught in the gdb.server/server-connect.exp test,
which, when run with the address sanitizer enabled, highlights the
write after free bug described above.

This commit resolves this issue by adding a new class specifically for
the purpose of managing the starting_up flag.  As well as setting, and
then clearing the starting_up flag, this new class increments, and
then decrements the reference count on the remote_target object.  This
prevents the remote_target from being deleted until after the flag has
been reset.

The gdb.server/server-connect.exp now runs cleanly with the address
sanitizer enabled.