Fix memcpy regression in PMK handling

Fix memcpy regression in PMK handling

The memcpy calls added for exposing the PMK from wpa_auth module could
end up trying to copy the same memory buffer on top of itself.
Overlapping memory areas are not allowed with memcpy, so this could
result in undefined behavior. Fix this by making the copies conditional
on the updated value actually coming from somewhere else.

Fixes: b08c9ad0c78d ("AP: Expose PMK outside of wpa_auth module")
Signed-off-by: Jouni Malinen <j@w1.fi>