mpeg4video: guarantee aspect_ratio_idc is 4bits

There was no guarantee from bs_read() that the result could not return 0xff itself, which would be like the case 0xf but with ar.sar_width and ar.sar_height uninitialized. This tells the compiler that everything is fine.

The alternative solution is to patch the vlc_bits.h file adding a macro for bs_read() doing:

#define bs_read(s, count) (bs_read(s, count) &
                           (uint32_t)(((uint64_t)1 << (uint64_t)(count+1)) - 1))

Unfortunately, using count from the macro makes is expansion-unsafe and using this and-operation inside bs_read() is not enough for the compiler to ensure that there is no more bits that count, despite the function being inline.

Fixes the following warnings:

In file included from ../../modules/packetizer/mpeg4video.c:43:
In function ‘h26x_get_aspect_ratio’,
    inlined from ‘ParseVOL.isra’ at ../../modules/packetizer/mpeg4video.c:403:9:
../../modules/packetizer/h26x_nal_common.h:117:18: warning: ‘ar.sar_width’ may be used uninitialized [-Wmaybe-uninitialized]
  117 |         *num = ar->sar_width;
      |                ~~^~~~~~~~~~~
../../modules/packetizer/mpeg4video.c: In function ‘ParseVOL.isra’:
../../modules/packetizer/mpeg4video.c:364:25: note: ‘ar.sar_width’ was declared here
  364 |     h26x_aspect_ratio_t ar;
      |                         ^~
In function ‘h26x_get_aspect_ratio’,
    inlined from ‘ParseVOL.isra’ at ../../modules/packetizer/mpeg4video.c:403:9:
../../modules/packetizer/h26x_nal_common.h:118:18: warning: ‘ar.sar_height’ may be used uninitialized [-Wmaybe-uninitialized]
  118 |         *den = ar->sar_height;
      |                ~~^~~~~~~~~~~~
../../modules/packetizer/mpeg4video.c: In function ‘ParseVOL.isra’:
../../modules/packetizer/mpeg4video.c:364:25: note: ‘ar.sar_height’ was declared here
  364 |     h26x_aspect_ratio_t ar;
      |                         ^~