wifi: (fixes #2332) fix buggy behaviour since commit 96b721d35158

RELEASE_NOTES

@@ -127,6 +127,7 @@ Bugs fixed
 - Bug 2321 - Wifi rate managers should not be triggered for each MPDU when A-MPDU is used
 - Bug 2327 - CWmin value selection for 802.11g is not compliant with the standard rules
 - Bug 2328 - changes to antenna configuration should update HT MCS set
+- Bug 2332 - Assert failed when using aggregation and two pairs of AP-STA
 
 Known issues
 ------------

src/wifi/model/mac-low.cc

@@ -846,8 +846,16 @@ MacLow::ReceiveError (Ptr<Packet> packet, double rxSnr, bool isEndOfFrame)
   if (isEndOfFrame == true && m_receivedAtLeastOneMpdu == true)
     {
       WifiMacHeader hdr;
-      MpduAggregator::DeaggregatedMpdus mpdu = MpduAggregator::Deaggregate (packet);
-      mpdu.begin ()->first->PeekHeader (hdr);
+      AmpduTag ampdu;
+      if (packet->RemovePacketTag (ampdu))
+        {
+          MpduAggregator::DeaggregatedMpdus mpdu = MpduAggregator::Deaggregate (packet);
+          mpdu.begin ()->first->PeekHeader (hdr);
+        }
+      else
+        {
+          packet->PeekHeader (hdr);
+        }
       if (hdr.GetAddr1 () != m_self)
         {
           NS_LOG_DEBUG ("hdr addr1 " << hdr.GetAddr1 () << "not for me (" << m_self << "); returning");