wil6210: check null pointer in _wil_cfg80211_merge_extra_ies

ies1 or ies2 might be null when code inside
_wil_cfg80211_merge_extra_ies access them.
Add explicit check for null and make sure ies1/ies2 are not
accessed in such a case.

Change-Id: I14908fcac0e1fc8e02e9460a86cc8e5edebb4192
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>