Commented out semicolon NOT ignored

If there is a semicolon within a single line comment "--", Npgsql
assumes that it marks the end of a statement and starts the next
statement (chunk) immediately after it, ignoring that this is still
commented out.
This commit ignores semicolons within single line comments.

I have created a test demonstrating the issue:
This will throw an exception although the command is valid (though meaningless in this case)
```csharp
[Test]
public void CommentedOutSemicolon()
{
  var command = new NpgsqlCommand("-- 1\n-- 2; abc\n-- 3;", Conn);
  command.ExecuteNonQuery();
}
```