multimedia/arcan: unbreak KMS support after cb7c09ae6a8e

$ arcan console
pid 12345 (arcan), jid 0, uid 1111: exited on signal 6 (no core dump - bad address)
$ tail -1 /var/log/messages
Sep 29 22:48:36 localhost arcan[12345]: stack overflow detected; terminated

(lldb) bt

• thread #2, name = 'arcan', stop reason = signal SIGABRT
  • frame #0: 0x00000008277faf5a libc.so.7`sys_kill at kill.S:4 frame #1: 0x00000008277fe361 libc.so.7`fail(msg="stack overflow detected; terminated") at stack_protector.c:120:8 frame #2: 0x00000008277fe2d0 libc.so.7`__stack_chk_fail at stack_protector.c:127:2 frame #3: 0x00000000002eb876 arcan`button_count(fd=5, bitn=1, got_mouse=0x000000082040a5cf, got_joy=0x000000082040a5ce) at event.c:0

(lldb) f 3
frame #3: 0x00000000002eb876 arcan`button_count(fd=5, bitn=1, got_mouse=0x000000082040a5cf, got_joy=0x000000082040a5ce) at event.c:0

844  #define bit_isset(ary, bit) (( ary[bit_longn(bit)] >> bit_ofs(bit)) & 1)
845  #define bit_count(x) ( ((x) - 1 ) / (sizeof(long) * 8 ) + 1 )
846

-> 847 static size_t button_count(int fd, size_t bitn, bool* got_mouse, bool* got_joy)

848  {
849          size_t count = 0;
850

PR: 274163
Reported by: Albin "a12l" Otterhäll