Fix a null Pointer exception bug for images missing an image height in the thumbnail.

Submitted by: Piyush Kapoor <pkapoor at adobe dot com>
Jira issue key: IMAGING-121



git-svn-id: https://svn.apache.org/repos/asf/commons/proper/imaging/trunk@1537588 13f79535-47bb-0310-9956-ffa450edef68