Fix compiler warning

[sjaeckel]

Repeated from 97edea3

src/encauth/ccm/ccm_add_nonce.c: In function ‘ccm_add_nonce’:
src/encauth/ccm/ccm_add_nonce.c:61:21: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
   61 |       ccm->PAD[x++] = (unsigned char)((len >> 24) & 255);
      |       ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./src/headers/tomcrypt.h:82,
                 from ./src/headers/tomcrypt_private.h:4,
                 from src/encauth/ccm/ccm_add_nonce.c:3:
./src/headers/tomcrypt_mac.h:410:24: note: at offset 16 into destination object ‘PAD’ of size 16
  410 |    unsigned char       PAD[16],              /* flags | Nonce N | l(m) */