[mdns-mdnssd] fix access of freed mHostsRef in Stop()

This commit updates `Stop()` method to clear the host and service
registrations `mHostRegistrations` and `mServiceRegistrations` before
de-allocating `mHostsRef`. This ensures that `mHostsRef` and all its
child `DNSRecordRef`s remain valid when the `DnssdHostRegistration`
destructor is called which accesses and uses these to update the
published AAAA (IPv6 Address) records.

This commit also updates how `kDNSServiceErr_ServiceNotRunning` is
handled. Upon getting this error we need to clear all existing
`ServiceRef`s immediately, so we de-allocate `mHostsRef` and then
call `Stop()` followed by `Start()`. This commit also adds a check
to ensure `mHostsRef` is valid in the `DnssdHostRegistration`
destructor. This way the `Stop()` works correctly during normal
stop process and also `kDNSServiceErr_ServiceNotRunning`

src/mdns/mdns_mdnssd.cpp

@@ -239,13 +239,10 @@ bool PublisherMDnsSd::IsStarted(void) const
 
 void PublisherMDnsSd::Stop(void)
 {
-    ServiceRegistrationMap serviceRegistrations;
-    HostRegistrationMap    hostRegistrations;
-
     VerifyOrExit(mState == State::kReady);
 
-    std::swap(mServiceRegistrations, serviceRegistrations);
-    std::swap(mHostRegistrations, hostRegistrations);
+    mServiceRegistrations.clear();
+    mHostRegistrations.clear();
 
     if (mHostsRef != nullptr)
     {
@@ -370,6 +367,13 @@ void PublisherMDnsSd::Process(const MainloopContext &aMainloop)
         if (error == kDNSServiceErr_ServiceNotRunning)
         {
             otbrLogWarning("Need to reconnect to mdnsd");
+
+            if (mHostsRef != nullptr)
+            {
+                DNSServiceRefDeallocate(mHostsRef);
+                mHostsRef = nullptr;
+            }
+
             Stop();
             Start();
             ExitNow();
@@ -403,6 +407,7 @@ PublisherMDnsSd::DnssdHostRegistration::~DnssdHostRegistration(void)
 {
     int dnsError;
 
+    VerifyOrExit(GetPublisher().mHostsRef != nullptr);
     VerifyOrExit(mServiceRef != nullptr);
 
     for (const auto &recordRefAndAddress : GetRecordRefMap())

src/mdns/mdns_mdnssd.hpp

@@ -156,6 +156,8 @@ class PublisherMDnsSd : public MainloopProcessor, public Publisher
         std::map<DNSRecordRef, Ip6Address>       &GetRecordRefMap() { return mRecordRefMap; }
 
     private:
+        PublisherMDnsSd &GetPublisher(void) { return *static_cast<PublisherMDnsSd *>(mPublisher); }
+
         DNSServiceRef mServiceRef;
 
     public: