Add input check to mp_pack and mp_pack_count

[MagicalTux]

mp_pack_count computes bits / ((size*8) - nails). When nails == size*8 the divisor is zero — a division by zero (SIGFPE on x86). mp_pack calls mp_pack_count unconditionally, so the same crash is reachable through mp_pack.

Like mp_unpack, these parameters can originate from an external format, making the bad combination reachable from untrusted input.

Fix: reject size == 0 and nails >= size*8 early in both functions (mp_pack_count returns 0, mp_pack returns MP_VAL). The check is placed before mp_count_bits in mp_pack_count.

Confirmed with UndefinedBehaviorSanitizer:

mp_pack_count.c:9:18: runtime error: division by zero

Reproducer: mp_pack_count(&a, 8, 1);