sd-login: always return two arrays of same length from sd_seat_get_sessions

sd_seat_get_sessions returns two arrays, that in principle should always match:
the session names and corresponding uids. The second array could be shorter only
if parsing or uid conversion fails. But in that case there is no way to tell
*which* uid is wrong, so they are *all* useless. It's better to simplify things and
just return an error if parsing fails.

man/sd_seat_get_active.xml

@@ -105,8 +105,9 @@
     one (<constant>NULL</constant> terminated) with the session
     identifiers of the sessions and one with the user identifiers of
     the Unix users the sessions belong to. An additional parameter may
-    be used to return the number of entries in the latter array. The
-    two arrays and the latter parameter may be passed as
+    be used to return the number of entries in the latter array. This
+    value is the same the return value, if the latter is nonnegative.
+    The two arrays and the last parameter may be passed as
     <constant>NULL</constant> in case these values need not to be
     determined. The arrays and the strings referenced by them need to
     be freed with the libc

src/libsystemd/sd-login/sd-login.c

@@ -723,7 +723,7 @@ _public_ int sd_seat_get_sessions(const char *seat, char ***sessions, uid_t **ui
 
                                 r = parse_uid(k, b + i);
                                 if (r < 0)
-                                        continue;
+                                        return r;
 
                                 i++;
                         }