Fix out-of-bounds write.

Author: bgrainger

src/MySqlConnector/MySql.Data.MySqlClient/MySqlBulkCopy.cs

@@ -160,7 +160,7 @@ await dbDataReader.ReadAsync(cancellationToken).ConfigureAwait(false) :
 						else
 							shouldAppendSeparator = true;
 
-						if (!WriteValue(connection, value, buffer.AsSpan(0, maxLength).Slice(outputIndex), out var bytesWritten))
+						if (outputIndex >= maxLength || !WriteValue(connection, value, buffer.AsSpan(0, maxLength).Slice(outputIndex), out var bytesWritten))
 						{
 							wroteRow = false;
 							break;