[ip6] update where HandlePayload() check message is not null (#9462)

This commit updates `Ip6::HandlePayload()` to check the `message`
pointer for null in the common flow, instead of in the `if()` block
where the message is cloned when `aMessageOwnership` is set to
`kCopyToUse`. If `aMessageOwnership` is `kTakeCustody` the `message`
is initialized as `&aMessage` already which cannot be `nullptr`.

This protects against code checker warnings that `message` may remain
null if the given `aMessageOwnership` enum value is not one of its
two defined enumerator values.

src/core/net/ip6.cpp

@@ -901,9 +901,11 @@ Error Ip6::HandlePayload(Header            &aIp6Header,
 
     if (aMessageOwnership == Message::kCopyToUse)
     {
-        VerifyOrExit((message = aMessage.Clone()) != nullptr, error = kErrorNoBufs);
+        message = aMessage.Clone();
     }
 
+    VerifyOrExit(message != nullptr, error = kErrorNoBufs);
+
     switch (aIpProto)
     {
 #if OPENTHREAD_CONFIG_TCP_ENABLE