tools: ynl: don't access uninitialized attr_space variable

If message contains unknown attribute and user passes
"--process-unknown" command line option, _decode() gets called with space
arg set to None. In that case, attr_space variable is not initialized
used which leads to following trace:

Traceback (most recent call last):
File "./tools/net/ynl/cli.py", line 77, in <module>
main()
File "./tools/net/ynl/cli.py", line 68, in main
reply = ynl.dump(args.dump, attrs)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "tools/net/ynl/lib/ynl.py", line 909, in dump
return self._op(method, vals, [], dump=True)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "tools/net/ynl/lib/ynl.py", line 894, in _op
rsp_msg = self._decode(decoded.raw_attrs, op.attr_set.name)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "tools/net/ynl/lib/ynl.py", line 639, in _decode
self._rsp_add(rsp, attr_name, None, self._decode_unknown(attr))
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "tools/net/ynl/lib/ynl.py", line 569, in _decode_unknown
return self._decode(NlAttrs(attr.raw), None)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "tools/net/ynl/lib/ynl.py", line 630, in _decode
search_attrs = SpaceAttrs(attr_space, rsp, outer_attrs)
^^^^^^^^^^
UnboundLocalError: cannot access local variable 'attr_space' where it is not associated with a value

Fix this by moving search_attrs assignment under the if statement
above it to make sure attr_space is initialized.

Fixes: bf8b832374fb ("tools/net/ynl: Support sub-messages in nested attribute spaces")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>