Check for sentinel value when setting HTTP/3 error code

If no error code has been set, `IProtocolErrorFeature.Error` will be `-1`.  If we pass that through verbatim, it will be caught by validation in the setter (ironically, of the same property on the same feature object), resulting in an exception and a Critical (but apparently benign) log message.

Fixes dotnet#57933

src/Servers/Kestrel/Core/src/Internal/Http3/Http3Connection.cs

@@ -546,7 +546,8 @@ public async Task ProcessRequestsAsync<TContext>(IHttpApplication<TContext> appl
                 }
 
                 // Complete
-                Abort(CreateConnectionAbortError(error, clientAbort), (Http3ErrorCode)_errorCodeFeature.Error, reason);
+                var errorCode = _errorCodeFeature.Error == -1 ? Http3ErrorCode.NoError : (Http3ErrorCode)_errorCodeFeature.Error;
+                Abort(CreateConnectionAbortError(error, clientAbort), errorCode, reason);
 
                 // Wait for active requests to complete.
                 while (_activeRequestCount > 0)