[ip6] adhere to style guide for var declarations in HandleDatagram() (#13320)

This is a small style guide fix in `Ip6::HandleDatagram()`, ensuring
all scoped stack variable declarations are placed together at the top
of the enclosing scope in which they are used.

src/core/net/ip6.cpp

@@ -1267,15 +1267,15 @@ exit:
 
 Error Ip6::HandleDatagram(OwnedPtr<Message> aMessagePtr, bool aIsReassembled, uint8_t aRecursionDepth)
 {
-    Error  error;
-    Header header;
-
-    VerifyOrExit(aRecursionDepth <= kMaxRecursionDepth, error = kErrorDrop);
+    Error   error;
+    Header  header;
     bool    receive;
     bool    forwardThread;
     bool    forwardHost;
     uint8_t nextHeader;
 
+    VerifyOrExit(aRecursionDepth <= kMaxRecursionDepth, error = kErrorDrop);
+
     SuccessOrExit(error = header.ParseFrom(*aMessagePtr));
 
     if (!aMessagePtr->IsOriginHostTrusted())