some gcc 15.2.1 warnings

As requested, some (harmless) gcc (15.2.1) warnings:

unrar.c: In function 'read_tables':
unrar.c:500:56: warning: writing 16 bytes into a region of size 4 [-Wstringop-overflow=]
  500 |                                         bit_length[i++]=0;
      |                                                        ^
unrar.c:450:23: note: at offset [16, 20] into destination object 'bit_length' of size 20
  450 |         unsigned char bit_length[BC];
      |                       ^
options.c: In function 'opt_init':
options.c:1030:27: warning: 'strchr' reading 1 or more bytes from a region of size 0 [-Wstringop-overread]
 1030 |                 char *e = strchr(s + 1, ':');
      |                           ^
In function 'check_huffman',
    inlined from 'check_rar' at rar_common.h:654:10:
rar_common.h:526:57: warning: writing 16 bytes into a region of size 4 [-Wstringop-overflow=]
  526 |                                         bit_length[i++] = 0;
      |                                                         ^
rar_common.h: In function 'check_rar':
rar_common.h:501:23: note: at offset [16, 20] into destination object 'bit_length' of size 20
  501 |         unsigned char bit_length[20];
      |                       ^
In function 'check_huffman',
    inlined from 'check_rar' at rar_common.h:654:10:
rar_common.h:526:57: warning: writing 16 bytes into a region of size 4 [-Wstringop-overflow=]
  526 |                                         bit_length[i++] = 0;
      |                                                         ^
rar_common.h: In function 'check_rar':
rar_common.h:501:23: note: at offset [16, 20] into destination object 'bit_length' of size 20
  501 |         unsigned char bit_length[20];
      |                       ^

Start by reproducing the reported warnings with GCC 15.2.1 and inspect unrar.c, options.c, and rar_common.h at the cited locations. Determine the intended bounds and input checks from the surrounding functions, then rebuild to confirm the listed warnings are resolved without changing behavior.