shared/dropin: fix assert for invalid drop-in

Don't try to show top level drop-in for non-existent units or when trying to
instantiate non-instantiated units:

$ systemctl cat nonexistent@.service
Assertion 'name' failed at src/shared/dropin.c:143, function unit_file_find_dirs(). Aborting.
$ systemctl cat systemd-journald@.service
Assertion 'name' failed at src/shared/dropin.c:143, function unit_file_find_dirs(). Aborting.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/TEST-15-DROPIN/test-dropin.sh b/test/TEST-15-DROPIN/test-dropin.sh
index fe424c1587..fd500c1a0b 100755
--- a/test/TEST-15-DROPIN/test-dropin.sh
+++ b/test/TEST-15-DROPIN/test-dropin.sh
@@ -419,9 +419,23 @@ EOF
     clear_services a b
 }
 
+test_invalid_dropins () {
+    echo "Testing invalid dropins..."
+    # Assertion failed on earlier versions, command exits unsuccessfully on later versions
+    systemctl cat nonexistent@.service || true
+    create_services a
+    systemctl daemon-reload
+    # Assertion failed on earlier versions, command exits unsuccessfully on later versions
+    systemctl cat a@.service || true
+    systemctl stop a
+    clear_services a
+    return 0
+}
+
 test_basic_dropins
 test_template_dropins
 test_alias_dropins
 test_masked_dropins
+test_invalid_dropins
 
 touch /testok