wil6210: check null pointer in _wil_cfg80211_merge_extra_ies

spos might be null and be accessed inside _wil_cfg80211_merge_extra_ies.
Add explicit check for null in the while condition statement and make
sure spos is not accessed in such a case.

Change-Id: I54143674ab7e2abbfa5704c2a315d790d8fff6b7
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>