app/main: Fix error handling for getting system dir list

The error wasn’t being propagated properly, leading to a NULL pointer
dereference.

Coverity CID: 1463075

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1267
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f