hdspmixer: fix null pointer dereference in HDSPMixerWindow:handler_cb

[cheldt]

Fixes following error:

HDSPMixer 1.11 - Copyright (C) 2003 Thomas Charbonnel <[email protected]>
This program comes with ABSOLUTELY NO WARRANTY
HDSPMixer is free software, see the file COPYING for details

Looking for RME cards:
Card 0: RME AIO Pro S/N 24102214 at 0x43d00000 irq 16
RME AIO found!
Card 1: HDA Intel PCH at 0x44710000 irq 214
Card 2: HDA NVidia at 0x43080000 irq 17
1 RME cards card found.
Restoring last presets used

Program received signal SIGSEGV, Segmentation fault.
handler_cb () at /home/snaut/dev/alsa-tools/hdspmixer/src/HDSPMixerWindow.cxx:278
278             if (fl_win->label()) {
(gdb) bt
#0  handler_cb () at /home/kenoby/dev/alsa-tools/hdspmixer/src/HDSPMixerWindow.cxx:278
#1  0x00007ffff7c5c791 in send_handlers () at /usr/src/debug/fltk/fltk-release-1.4.4/src/Fl.cxx:935
#2  Fl::handle_ () at /usr/src/debug/fltk/fltk-release-1.4.4/src/Fl.cxx:1620
#3  Fl::handle_ () at /usr/src/debug/fltk/fltk-release-1.4.4/src/Fl.cxx:1384
#4  0x00007ffff6dfb2ee in ffi_call_unix64 () at ../src/x86/unix64.S:104
#5  0x00007ffff6df79f8 in ffi_call_int () at ../src/x86/ffi64.c:676
#6  0x00007ffff6dfa6ce in ffi_call () at ../src/x86/ffi64.c:713
#7  0x00007ffff732d47a in wl_closure_invoke () at ../wayland-1.24.0/src/connection.c:1241
#8  0x00007ffff732e2e9 in dispatch_event () at ../wayland-1.24.0/src/wayland-client.c:1707
#9  0x00007ffff732e6bb in dispatch_queue () at ../wayland-1.24.0/src/wayland-client.c:1853
#10 wl_display_dispatch_queue_pending () at ../wayland-1.24.0/src/wayland-client.c:2190
#11 0x00007ffff7332776 in wl_display_dispatch_queue_timeout () at ../wayland-1.24.0/src/wayland-client.c:2096
#12 0x00007ffff7332860 in wl_display_dispatch_queue () at ../wayland-1.24.0/src/wayland-client.c:2163
#13 0x00007ffff7d017b2 in sync_done () at /usr/src/debug/fltk/fltk-release-1.4.4/src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx:1389
#14 0x00007ffff6dfb2ee in ffi_call_unix64 () at ../src/x86/unix64.S:104
#15 0x00007ffff6df79f8 in ffi_call_int () at ../src/x86/ffi64.c:676
#16 0x00007ffff6dfa6ce in ffi_call () at ../src/x86/ffi64.c:713
#17 0x00007ffff732d47a in wl_closure_invoke () at ../wayland-1.24.0/src/connection.c:1241
#18 0x00007ffff732e2e9 in dispatch_event () at ../wayland-1.24.0/src/wayland-client.c:1707
#19 0x00007ffff732e6bb in dispatch_queue () at ../wayland-1.24.0/src/wayland-client.c:1853
#20 wl_display_dispatch_queue_pending () at ../wayland-1.24.0/src/wayland-client.c:2190
#21 0x00007ffff7332776 in wl_display_dispatch_queue_timeout () at ../wayland-1.24.0/src/wayland-client.c:2096
#22 0x00007ffff7332860 in wl_display_dispatch_queue () at ../wayland-1.24.0/src/wayland-client.c:2163
#23 0x00007ffff7d03561 in Fl_Wayland_Screen_Driver::open_display_platform () at /usr/src/debug/fltk/fltk-release-1.4.4/src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx:1426
#24 Fl_Wayland_Screen_Driver::open_display_platform () at /usr/src/debug/fltk/fltk-release-1.4.4/src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx:1406
#25 0x00007ffff7ca6da7 in Fl_Screen_Driver::open_display () at /usr/src/debug/fltk/fltk-release-1.4.4/src/Fl_Screen_Driver.cxx:605
#26 Fl_Screen_Driver::open_display () at /usr/src/debug/fltk/fltk-release-1.4.4/src/Fl_Screen_Driver.cxx:600
#27 0x00007ffff7d03d14 in Fl_Wayland_Screen_Driver::get_system_colors () at /usr/src/debug/fltk/fltk-release-1.4.4/src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx:1708
#28 0x00007ffff7cec929 in Fl_Window::show () at /usr/src/debug/fltk/fltk-release-1.4.4/src/Fl_arg.cxx:290
#29 0x000055555557b8aa in Fl_Double_Window::show () at /usr/include/FL/Fl_Double_Window.H:36
#30 main () at /home/kenoby/dev/alsa-tools/hdspmixer/src/hdspmixer.cxx:120

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