[srp] clear name on ExtractLabels() failure in AdvertisingProxy (openthread#13061)

This commit updates `AdvertisingProxy::CopyNameAndRemoveDomain()` to
properly handle potential errors returned by `Dns::Name::ExtractLabels()`.

Previously, any error returned by `ExtractLabels()` was ignored, which
could leave the output name buffer in an indeterminate state. With this
change, if extracting the labels fails, the name buffer is explicitly
cleared by setting its first character to `kNullChar`.

This prevents subsequent code from using uninitialized or partially
written data in the event of a parsing or buffer size error.

Author: abtink

src/core/net/srp_advertising_proxy.cpp

@@ -1152,7 +1152,10 @@ void AdvertisingProxy::UnregisterKey(const char *aName, const char *aServiceType
 
 void AdvertisingProxy::CopyNameAndRemoveDomain(DnsName &aName, const char *aFullName)
 {
-    IgnoreError(Dns::Name::ExtractLabels(aFullName, Get<Server>().GetDomain(), aName, sizeof(aName)));
+    if (Dns::Name::ExtractLabels(aFullName, Get<Server>().GetDomain(), aName, sizeof(aName)) != kErrorNone)
+    {
+        aName[0] = kNullChar;
+    }
 }
 
 void AdvertisingProxy::HandleRegistered(otInstance *aInstance, otPlatDnssdRequestId aRequestId, otError aError)