project_release adds release-related links to project nodes that don't have releases enabled

I noticed this while investigating #325444-20: Make usage statistics EVEN MORE visible...

A little CVS archeology shows that the bug was introduced by #218571: add hook_project_page_link_alter(), implement to add security link to all project pages and none of us noticed. Basically, these links were previously generated by theme_project_release_project_download_table(), which immediately returns no data if $node->releases isn't set. When the links were moved into the project_release implementation of hook_project_page_link_alter(), the same check was lost.

Should be trivial to fix, stay tuned.