suspend: support logind not sending PrepareForSleep(true)

elogind sends PrepareForSleep(false) when resuming but doesn't necessarily
send PrepareForSleep(true) when sleeping. This means we didn't necessarily
drop our previous inhibitor when it's time to take a new one. So instead of
asserting that we dropped our previous inhibitor, just reuse it.

src/mm-sleep-monitor-systemd.c

@@ -114,7 +114,12 @@ inhibit_done (GObject      *source,
 static void
 take_inhibitor (MMSleepMonitor *self)
 {
-    g_assert (self->inhibit_fd == -1);
+    if (self->inhibit_fd != -1) {
+        // logind didn't send us a `PrepareForSleep(true)` so we didn't drop our previous inhibitor.
+        // Just reuse it.
+        mm_obj_warn (self, "logind did not send sleep notification! Taking over existing inhibitor.");
+        return;
+    }
 
     mm_obj_dbg (self, "taking systemd sleep inhibitor");
     g_dbus_proxy_call_with_unix_fd_list (self->sd_proxy,