NPE in BlockBox

Just added a null check in calcCanvasLocation.  Exception was happening when a paginated table with headers that had a row with a floating div in it was spanning pages.  Just adding the null check seemed to cause the page to display fine.

Sample that causes the exception:
```HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
	<style type='text/css'>
		.left { float: left; }
		table { -fs-table-paginate: paginate; }
	</style>
</head>
<body>	
<div style="height:930px">&nbsp;</div>
<table>
	<thead>
		<tr>
			<th>test header</th>
		</tr>
	</thead>
	<tbody>					
		<tr>
			<td>
				<div class="left">test cell</div>
			</td>
		</tr>					
	</tbody>
</table>
</body>
</html>
```