Throw better exception for protocol violation.

As soon as a malformed column_count packet is detected, throw a descriptive exception (instead of failing at an arbitrary point later during deserialization).

Author: bgrainger

src/MySqlConnector/MySqlClient/Results/ResultSet.cs

@@ -79,6 +79,9 @@ public async Task<ResultSet> ReadResultSetHeaderAsync(IOBehavior ioBehavior)
 					{
 						var reader = new ByteArrayReader(payload.ArraySegment);
 						var columnCount = (int) reader.ReadLengthEncodedInteger();
+						if (reader.BytesRemaining != 0)
+							throw new MySqlException("Unexpected data at end of column_count packet; see https://github.com/mysql-net/MySqlConnector/issues/324");
+
 						ColumnDefinitions = new ColumnDefinitionPayload[columnCount];
 						m_dataOffsets = new int[columnCount];
 						m_dataLengths = new int[columnCount];