[mdns] handle allocation failure and fix use-after-release (#11854)

This commit changes the `RxMsgEntry` allocation check from `OT_ASSERT`
to `VerifyOrExit`. This ensures that a heap allocation failure for a
multi-packet RX message results in the message being gracefully
dropped, allowing network operation to continue instead of
asserting.

Additionally, `AddNew()` now removes any existing multi-packet message
entries from the same sender before attempting to allocate a new
`RxMsgEntry`. This helps to reclaim resources and prevent stale
entries.

This change also fixes a use-after-release bug by ensuring
that the `aRxMessagePtr` (an `OwnedPtr`) is not accessed after its
ownership is transferred by the `newEntry->Add(aRxMessagePtr)`
call.

src/core/net/mdns.cpp

@@ -4859,16 +4859,22 @@ exit:
 
 void Core::MultiPacketRxMessages::AddNew(OwnedPtr<RxMessage> &aRxMessagePtr)
 {
-    RxMsgEntry *newEntry = RxMsgEntry::Allocate(GetInstance());
+    RxMsgEntry *newEntry;
 
-    OT_ASSERT(newEntry != nullptr);
-    newEntry->Add(aRxMessagePtr);
-
-    // First remove an existing entries matching same sender
+    // First remove existing entries matching same sender
     // before adding the new entry to the list.
 
     mRxMsgEntries.RemoveMatching(aRxMessagePtr->GetSenderAddress());
+
+    newEntry = RxMsgEntry::Allocate(GetInstance());
+    VerifyOrExit(newEntry != nullptr);
+
+    newEntry->Add(aRxMessagePtr);
+
     mRxMsgEntries.Push(*newEntry);
+
+exit:
+    return;
 }
 
 void Core::MultiPacketRxMessages::HandleTimer(void)