logind: simplify code

Follow-up for 4885d7490b23e08d8444e5a68927ce9ce8727e5a.

(cherry picked from commit e5c09aad375551b9db499703ab7eb123d408ba16)

Resolves: #2209328

src/login/logind-session.c

@@ -349,12 +349,11 @@ int session_save(Session *s) {
 }
 
 static int session_load_devices(Session *s, const char *devices) {
-        const char *p;
         int r = 0;
 
         assert(s);
 
-        for (p = devices;;) {
+        for (const char *p = devices;;) {
                 _cleanup_free_ char *word = NULL;
                 SessionDevice *sd;
                 dev_t dev;
@@ -531,7 +530,7 @@ int session_load(Session *s) {
                         s->class = c;
         }
 
-        if (state && streq(state, "closing"))
+        if (streq_ptr(state, "closing"))
                 s->stopping = true;
 
         if (s->fifo_path) {
@@ -1073,11 +1072,8 @@ int session_set_display(Session *s, const char *display) {
         assert(s);
         assert(display);
 
-        if (streq(s->display, display))
-                return 0;
-
         r = free_and_strdup(&s->display, display);
-        if (r < 0)
+        if (r <= 0)  /* 0 means the strings were equal */
                 return r;
 
         session_save(s);