This is a fix for issue 0002808 (Current project removal causes error)

• core/helper_api.php
  (helper_set_current_project): new function

• set_project.php
  use helper_set_current_project()

• manage_proj_delete.php
  use helper_set_current_project() to set the current project to All Projects when you try to delete the current project

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1703 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9