[show/fgnhg] Fix crash bug

[jleveque]

- What I did

Fix crashes like the following:

Traceback (most recent call last):
  File "/usr/local/bin/show", line 10, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/show/fgnhg.py", line 56, in active_hops
    click.echo(tabulate(table, header, tablefmt="grid"))
UnboundLocalError: local variable 'header' referenced before assignment

- How I did it

Define table header outside the loop which processes the rows

- How to verify it

Run the following commands, confirm they don't crash like above:

show fgnhg active-hops
show fgnhg hash-view

Example output with this fix:

admin@sonic:~$ show fgnhg active-hops
+-----------------+--------------------+
| FG_NHG_PREFIX   | Active Next Hops   |
+=================+====================+
+-----------------+--------------------+

admin@sonic:~$ show fgnhg hash-view
+-----------------+------------+----------------+
| FG_NHG_PREFIX   | Next Hop   | Hash buckets   |
+=================+============+================+
+-----------------+------------+----------------+

[kktheballer]

retest this please

[jleveque]

Retest this please