Fixed 0003537: Rewriting custom values with 0 when updating a bug.

M bug_update.php

• If a custom field is not posted, do not set it, rather than re-setting it to the default value.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2326 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9