[cli] fix infinite loop on networkdiagnostic (#4882)

Reproduce by issuing networkdiagnostic get ff03::1 0 1 on one router.

src/core/thread/network_diagnostic.cpp

@@ -1029,7 +1029,7 @@ exit:
     if (error == OT_ERROR_NONE)
     {
         aNetworkDiagTlv.mType = tlv.GetType();
-        aIterator             = offset - message.GetOffset();
+        aIterator             = static_cast<uint16_t>(offset - message.GetOffset() + tlv.GetSize());
     }
     return error;
 }