[mdns-avahi] allow UnsubscribeService/Host() to remove non-present …

…entries

This commit adds a similar change to openthread#2206 but for Avahi example.
It updates `UnsubscribeService/Host()` to allow them to be
called with service or host name that may not be present in the
list. This situation may happen if duplicate queries are received
for the same names.

src/mdns/mdns_avahi.cpp

@@ -1047,7 +1047,7 @@ void PublisherAvahi::UnsubscribeService(const std::string &aType, const std::str
                           return aService->mType == aType && aService->mInstanceName == aInstanceName;
                       });
 
-    assert(it != mSubscribedServices.end());
+    VerifyOrExit(it != mSubscribedServices.end());
 
     {
         std::unique_ptr<ServiceSubscription> service = std::move(*it);
@@ -1106,7 +1106,7 @@ void PublisherAvahi::UnsubscribeHost(const std::string &aHostName)
         mSubscribedHosts.begin(), mSubscribedHosts.end(),
         [&aHostName](const std::unique_ptr<HostSubscription> &aHost) { return aHost->mHostName == aHostName; });
 
-    assert(it != mSubscribedHosts.end());
+    VerifyOrExit(it != mSubscribedHosts.end());
 
     {
         std::unique_ptr<HostSubscription> host = std::move(*it);