iwlwifi: check allocated pointer when allocating conf_tlvs

We were erroneously checking the length of the tlv instead of checking
the pointer returned by kmemdup() when allocating dbg_conf_tlv[].
This was probably a typo.  Fix it by checking the returned pointer
instead of the length.

Reported-by: Markus Elfring <Markus.Elfring@web.de>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200306151128.06e00e6e980f.I9a890ce83493b79892a5f690d12016525317fa7e@changeid