wifi: cfg80211: reject empty PMSR peer lists

wifi: cfg80211: reject empty PMSR peer lists

A PMSR request with an empty peers array is not a useful request and
weakens the cfg80211-to-driver contract by allowing start_pmsr() with
no target peer.

Reject empty peer lists before allocating the request object or calling
into the driver.

Fixes: 9bb7e0f24e7e7 ("cfg80211: add peer measurement with FTM initiator API")
Assisted-by: Codex:gpt-5.5
Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Zhao Li <enderaoelyther@gmail.com>
Link: https://patch.msgid.link/20260612133717.93783-2-enderaoelyther@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>