OWE: Clean up pointer check in a testing code path

OWE: Clean up pointer check in a testing code path

Check wpa_auth_write_assoc_resp_owe() return value to keep static
analyzers happier. The code path where this could happen is not really
reachable due to the separate hapd->conf->own_ie_override check and
wpa_auth_write_assoc_resp_owe() returning NULL only in an error case in
the override path. Furthermore, clean up the pointer return value to use
a proper pointer (NULL vs. 0).

Signed-off-by: Ashok Ponnaiah <aponnaia@codeaurora.org>