lib, zebra: fix exit commands

lib, zebra: fix exit commands

If a command is not marked as `YANG`-converted, the current command
batching buffer is flushed before executing the command. We shouldn't
flush the buffer when executing an `exit` command. It should only be
flushed if the next command is not `YANG`-converted, which is checked by
the command itself, not the previous `exit`.

Fixes #15706.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>