hostapd: Fix a regression in TKIP countermeasures processing

hostapd: Fix a regression in TKIP countermeasures processing

Commit 296a34f0c1730416bf2a61ab78690be43d82a3c0 changed hostapd to
remove the internal STA entry at the beginning of TKIP countermeasures.
However, this did not take into account the case where this is triggered
by an EAPOL-Key error report from a station. In such a case, WPA
authenticator state machine may continue processing after having
processed the error report. This could result in use of freed memory.
Fix this by stopping WPA processing if the STA entry got removed.

Signed-hostap: Jouni Malinen <j@w1.fi>