netplay: only reinit preempt frames if core is running

deinit_netplay unconditionally calls preempt_init at the bottom
of its body. This is appropriate when netplay is being toggled
off mid-session - we want to restore preempt frames so the user
gets runahead back. But deinit_netplay is also called from two
shutdown contexts where the call is wasted work or worse:

  - RARCH_CTL_MAIN_DEINIT (retroarch.c:8511) at full app
    shutdown: preempt frames will be torn down moments later
    by CMD_EVENT_CORE_DEINIT's preempt_deinit.

  - retroarch_main_init's error: label (retroarch.c:8431) at
    init failure: same teardown follows.

Beyond the wasted allocation, preempt_init at runahead.c:1501
synchronously calls runloop_st->current_core.retro_run() when
frame_count is 0. Firing a synchronous retro_run from inside
a deinit chain - while RARCH_CTL_MAIN_DEINIT is on the stack
or while we're unwinding from a failed init - is a surprising
side effect that any future change to retro_run could turn
into a bug.

Gate the call on RUNLOOP_FLAG_CORE_RUNNING. The flag is cleared
at the top of CMD_EVENT_UNLOAD_CORE (retroarch.c:3749), so the
shutdown paths see it unset and skip preempt_init. The mid-
session toggle paths (CMD_EVENT_NETPLAY_DEINIT from the netplay
menu actions in CMD_EVENT_NETPLAY_ENABLE_HOST etc.) see it set
and call preempt_init as before.

Author: LibretroAdmin

network/netplay/netplay_frontend.c

@@ -9070,8 +9070,22 @@ void deinit_netplay(void)
                                 |   NET_DRIVER_ST_FLAG_NETPLAY_IS_CLIENT);
 
 #if HAVE_RUNAHEAD
-      /* Reinitialize preemptive frames if enabled */
-      preempt_init(runloop_state_get_ptr());
+      /* Reinitialize preemptive frames if we're disabling
+       * netplay mid-session. Skip if the core isn't running:
+       * deinit_netplay is also called from the full shutdown
+       * chain (RARCH_CTL_MAIN_DEINIT at retroarch.c:8511) and
+       * from the retroarch_main_init error: label, where the
+       * subsequent CMD_EVENT_CORE_DEINIT will tear down
+       * preempt frames anyway via preempt_deinit. Allocating
+       * a fresh preempt frame buffer just to free it seconds
+       * later is wasted work, and the synchronous retro_run
+       * path inside preempt_init at runahead.c:1501 is a
+       * surprising side effect to fire from within a deinit
+       * chain. RUNLOOP_FLAG_CORE_RUNNING is cleared at the
+       * top of CMD_EVENT_UNLOAD_CORE (retroarch.c:3749),
+       * making it the right discriminator. */
+      if (runloop_state_get_ptr()->flags & RUNLOOP_FLAG_CORE_RUNNING)
+         preempt_init(runloop_state_get_ptr());
 #endif
 
       if (net_st->core_netpacket_interface