multimedia/mpv: unbreak --vo=gpu-next with LCMS2=off after bc9f898a9c83

$ mpv --vo=gpu-next foo.mp4
[...]
Segmentation fault
(lldb) bt

• thread #10, name = 'mpv/vo', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
  • frame #0: 0x000000083b4c8e84 libc.so.7`strlen + 84 frame #1: 0x000000083b4996bd libc.so.7`vsscanf + 173 frame #2: 0x000000083b48c72d libc.so.7`sscanf + 141 frame #3: 0x00000000004c0dd4 mpv`gl_parse_3dlut_size(arg=0x0000000000000000, p1=0x000000088fcf7d3c, p2=0x000000088fcf7d38, p3=0x000000088fcf7d34) at lcms.h:45:9 frame #4: 0x00000000004c085b mpv`update_icc_opts(p=0x00000008792f9b10, opts=0x0000000891370050) at vo_gpu_next.c:1619:5 frame #5: 0x00000000004c00ef mpv`update_render_options(vo=0x0000000875b38910) at vo_gpu_next.c:1863:5 frame #6: 0x00000000004bde05 mpv`preinit(vo=0x0000000875b38910) at vo_gpu_next.c:1426:5 frame #7: 0x000000000048cc99 mpv`vo_thread(ptr=0x0000000875b38910) at vo.c:1088:13 frame #8: 0x000000083bc4783a libthr.so.3`___lldb_unnamed_symbol556 + 314