ui: fix crash on serial reset, during init

For ex, when resetting the xlnx-zcu102 machine:

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason =
EXC_BAD_ACCESS (code=1, address=0x50)
   * frame #0: 0x10020a740 gd_vc_send_chars(vc=0x000000000) at
gtk.c:1759:41 [opt]
     frame #1: 0x100636264 qemu_chr_fe_accept_input(be=<unavailable>) at
char-fe.c:159:9 [opt]
     frame #2: 0x1000608e0 cadence_uart_reset_hold [inlined]
uart_rx_reset(s=0x10810a960) at cadence_uart.c:158:5 [opt]
     frame #3: 0x1000608d4 cadence_uart_reset_hold(obj=0x10810a960) at
cadence_uart.c:530:5 [opt]
     frame #4: 0x100580ab4 resettable_phase_hold(obj=0x10810a960,
opaque=0x000000000, type=<unavailable>) at resettable.c:0 [opt]
     frame #5: 0x10057d1b0 bus_reset_child_foreach(obj=<unavailable>,
cb=(resettable_phase_hold at resettable.c:162), opaque=0x000000000,
type=RESET_TYPE_COLD) at bus.c:97:13 [opt]
     frame #6: 0x1005809f8 resettable_phase_hold [inlined]
resettable_child_foreach(rc=0x000060000332d2c0, obj=0x0000600002c1c180,
cb=<unavailable>, opaque=0x000000000, type=RESET_TYPE_COLD) at
resettable.c:96:9 [opt]
     frame #7: 0x1005809d8 resettable_phase_hold(obj=0x0000600002c1c180,
opaque=0x000000000, type=RESET_TYPE_COLD) at resettable.c:173:5 [opt]
     frame #8: 0x1005803a0
resettable_assert_reset(obj=0x0000600002c1c180, type=<unavailable>) at
resettable.c:60:5 [opt]
     frame #9: 0x10058027c resettable_reset(obj=0x0000600002c1c180,
type=RESET_TYPE_COLD) at resettable.c:45:5 [opt]

While the chardev is created early, the VirtualConsole is associated
after, during qemu_init_displays().

Signed-off-by: Marc-André Lureau <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <[email protected]>

ui/gtk.c

@@ -1784,7 +1784,9 @@ static void gd_vc_chr_accept_input(Chardev *chr)
     VCChardev *vcd = VC_CHARDEV(chr);
     VirtualConsole *vc = vcd->console;
 
-    gd_vc_send_chars(vc);
+    if (vc) {
+        gd_vc_send_chars(vc);
+    }
 }
 
 static void gd_vc_chr_set_echo(Chardev *chr, bool echo)