Fixed 0003506: PHP error if "duplicate id" refers to a bug that does not exist.

M core/string_api.php

• (string_get_bug_view_link) If the bug does not exist, just return the number without hyperlink or title.
  This avoids calling bug_get_field() which causes the error.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2285 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9