fix discarded-qualifiers warning with memchr on GCC 16

[fcelda]

Fixes the following warning on GCC 16:

bcd-amalgamated.c:1406:11: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 1406 |         e = memchr(value, '\0', packet->length - k_l - 1);