dissect: guard against ssize_t overflow in LUKS2 header parser

dissect: guard against ssize_t overflow in LUKS2 header parser

The json_len variable is ssize_t, but the subtraction
be64toh(header.hdr_len) - LUKS2_FIXED_HDR_SIZE can yield a value
exceeding SSIZE_MAX when hdr_len is a large crafted value. This causes
signed integer overflow and a subsequent oversized malloc() that fails
with -ENOMEM, producing a misleading out-of-memory error.

Add an explicit check against SSIZE_MAX before the cast to ssize_t.