Fix PASESession and CASESession use-after-free

PASESession and CASESession clear exchange contexts for which they are
registered as delegate without closing them.  This leads to a situation
where exchanges can callback into these objects after they are freed.
This commit fixes the problem by setting the exchange context delegates
to nullptr before clearing the exchange contexts.

Fixes project-chip#13146

src/protocols/secure_channel/CASESession.cpp

@@ -255,6 +255,7 @@ void CASESession::OnResponseTimeout(ExchangeContext * ec)
     mDelegate->OnSessionEstablishmentError(CHIP_ERROR_TIMEOUT);
     // Null out mExchangeCtxt so that Clear() doesn't try closing it.  The
     // exchange will handle that.
+    mExchangeCtxt->SetDelegate(nullptr);
     mExchangeCtxt = nullptr;
     Clear();
 }
@@ -684,6 +685,7 @@ CHIP_ERROR CASESession::HandleSigma2Resume(System::PacketBufferHandle && msg)
     mCASESessionEstablished = true;
 
     // Forget our exchange, as no additional messages are expected from the peer
+    mExchangeCtxt->SetDelegate(nullptr);
     mExchangeCtxt = nullptr;
 
     // Call delegate to indicate session establishment is successful
@@ -1118,6 +1120,7 @@ CHIP_ERROR CASESession::HandleSigma3(System::PacketBufferHandle && msg)
     mCASESessionEstablished = true;
 
     // Forget our exchange, as no additional messages are expected from the peer
+    mExchangeCtxt->SetDelegate(nullptr);
     mExchangeCtxt = nullptr;
 
     // Call delegate to indicate session establishment is successful

src/protocols/secure_channel/PASESession.cpp

@@ -346,6 +346,7 @@ void PASESession::OnResponseTimeout(ExchangeContext * ec)
     mDelegate->OnSessionEstablishmentError(CHIP_ERROR_TIMEOUT);
     // Null out mExchangeCtxt so that Clear() doesn't try closing it.  The
     // exchange will handle that.
+    mExchangeCtxt->SetDelegate(nullptr);
     mExchangeCtxt = nullptr;
     Clear();
 }
@@ -824,6 +825,7 @@ CHIP_ERROR PASESession::HandleMsg3(System::PacketBufferHandle && msg)
     mPairingComplete = true;
 
     // Forget our exchange, as no additional messages are expected from the peer
+    mExchangeCtxt->SetDelegate(nullptr);
     mExchangeCtxt = nullptr;
 
     // Call delegate to indicate pairing completion
@@ -843,6 +845,7 @@ void PASESession::OnSuccessStatusReport()
     mPairingComplete = true;
 
     // Forget our exchange, as no additional messages are expected from the peer
+    mExchangeCtxt->SetDelegate(nullptr);
     mExchangeCtxt = nullptr;
 
     // Call delegate to indicate pairing completion