[net-diags] fix GetNextDiagTlv() when skipping over unknown TLVs (#8957)

This commit fixes `GetNextDiagTlv()` to correctly return error
`kErrorNotFound` if the message happens to end with an unknown TLV.
We ensure to set the `error` after the end of `while()` loop. With
the previous code we could return `kErrorNone` (since we would
successfully read and skip over the unknown TLV which would set
`error`).

src/core/thread/network_diagnostic.cpp

@@ -572,7 +572,7 @@ static inline void ParseMacCounters(const MacCountersTlv &aMacCountersTlv, otNet
 
 Error Client::GetNextDiagTlv(const Coap::Message &aMessage, Iterator &aIterator, TlvInfo &aTlvInfo)
 {
-    Error    error  = kErrorNotFound;
+    Error    error;
     uint16_t offset = (aIterator == 0) ? aMessage.GetOffset() : aIterator;
 
     while (offset < aMessage.GetLength())
@@ -774,10 +774,13 @@ Error Client::GetNextDiagTlv(const Coap::Message &aMessage, Iterator &aIterator,
             // Exit if a TLV is recognized and parsed successfully.
             aTlvInfo.mType = tlv.GetType();
             aIterator      = offset;
+            error          = kErrorNone;
             ExitNow();
         }
     }
 
+    error = kErrorNotFound;
+
 exit:
     return error;
 }