fix errors found through static code analysis, fixes #4562

app/controllers/admin/courses.php

app/controllers/admin/tree.php

app/controllers/course/admission.php

app/controllers/course/basicdata.php

app/controllers/course/feedback.php

app/controllers/debugbar.php

app/controllers/institute/basicdata.php

app/controllers/resources/ajax.php

lib/classes/AdminCourseFilter.php

lib/classes/Avatar.php

lib/classes/JsonApi/Middlewares/DangerousRouteHandler.php

lib/classes/JsonApi/RouteMap.php

...sses/JsonApi/Routes/Courseware/Rel/SolversOfTaskGroup.php

lib/classes/JsonApi/Routes/Feedback/RatingHelper.php

lib/classes/JsonApi/Routes/Files/FileRefsContentShow.php

lib/classes/JsonApi/Routes/Holidays/HolidaysShow.php

...sses/JsonApi/Routes/RangeTree/ChildrenOfRangeTreeNode.php

lib/classes/OAuth2/NegotiatesWithPsr7.php

lib/classes/SQLQuery.php

lib/classes/admission/CourseSet.php