In src/lib/krb5/krb/authdata.c context is NULL and is dereferenced:

static krb5_error_code
k5_ad_size(krb5_context kcontext,
           krb5_authdata_context context,
           krb5_flags flags,
           size_t *sizep)
{
    int i;
    krb5_error_code code = 0;

    *sizep += sizeof(krb5_int32); /* count */

    for (i = 0; i < context->n_modules; i++) {


The back trace is:

#0  0x0045dfcf in k5_ad_size (kcontext=0x8054af8, context=0x0, flags=15,

    sizep=0xbffff078)
    at krb5/src/lib/krb5/krb/authdata.c:162
#1  0x0045f7a2 in krb5_authdata_context_size (kcontext=0x8054af8,
ptr=0x0, 
    sizep=0xbffff078)
    at krb5/src/lib/krb5/krb/authdata.c:1131 (line 1067 in your trunk)
#2  0x00484310 in krb5_size_opaque (kcontext=0x8054af8,
odtype=-1760647364, 
    arg=0x0, sizep=0xbffff078)
    at krb5/src/lib/krb5/krb/serialize.c:104 (line 105 in your trunk)
#3  0x006ed9c3 in kg_ctx_size (kcontext=0x8054af8, arg=0x8053700, 
    sizep=0xbffff0b4)
    at krb5/src/lib/gssapi/krb5/ser_sctx.c:361