Validate the requested image exists within the project

[cpeel]

Diff links from other pages should always have valid $image values for a project, but if the user is adjusting the URL by hand the image they request might not exist in the project.

This validation ensures that the rest of the page works instead of these from the php_errors file:

29-Sep-2026 23:02:12 America/New_York
    PHP Warning:  foreach() argument must be of type array|object, null given in /data/htdocs/c/tools/project_manager/diff.php on line 332
    TypeError: get_navigation(): Argument #5 ($L_user) must be of type string, null given, called in /data/htdocs/c/tools/project_manager/diff.php on line 114 and defined in /data/htdocs/c/tools/project_manager/diff.php:185
Stack trace:
#0 /data/htdocs/c/tools/project_manager/diff.php(114): get_navigation()
#1 {main}

[bpfoley]

This makes the if (!$project->pages_table_exists) { ... } below redundant (get_project_page checks that first, and throws an exception).