install: fix incorrect 'Access denied' message with a non-existent unit

With "systemctl is-enabled non-existent.service"
_UNIT_FILE_STATE_INVALID (-1) was wrongly interpreted as -errno.
Return -ENOENT in this case.

https://bugzilla.redhat.com/show_bug.cgi?id=766579

src/install.c

@@ -1571,10 +1571,10 @@ UnitFileState unit_file_get_state(
                 }
 
                 if (lstat(path, &st) < 0) {
+                        r = -errno;
                         if (errno == ENOENT)
                                 continue;
 
-                        r = -errno;
                         goto finish;
                 }