Fix NULL strcmp and misaligned load on malformed tracks

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Fixes the two undefined behaviors reported in #99.

1. NULL pointer passed to strcmp (MP4File::GenerateTracks)

if (!strcmp(pTypeProperty->GetValue(), MP4_HINT_TRACK_TYPE)) {

A malformed trak/mdia/hdlr can leave the handlerType string property with a NULL value, so strcmp(NULL, ...) is UB. UBSan reports null pointer passed as argument 1, which is declared to never be null. Guard with a NULL check first:

const char* handlerType = pTypeProperty->GetValue();
if (handlerType && !strcmp(handlerType, MP4_HINT_TRACK_TYPE)) {

2. Misaligned load in STRTOINT32/INT32TOSTR

return MP4V2_NTOHL(*(uint32_t *)s);

When MP4V2_INTSTRING_ALIGNMENT is not defined, STRTOINT32 dereferences *(uint32_t*)s. Atom-type strings reached through ATOMID() (e.g. in MP4Atom::ReadChildAtoms) are not guaranteed 4-byte aligned, so this is a misaligned load. The fix makes the memcpy path unconditional (it was already the ARM-safe path), avoiding the UB on all platforms:

uint32_t tmp;
memcpy( &tmp, s, sizeof(tmp) );
return MP4V2_NTOHL( tmp );

INT32TOSTR is made consistent (always memcpy).

Verified: the reproducer from #99 triggers null pointer passed as argument 1 on the unfixed build; with this change it parses cleanly (exit 0, no sanitizer report) under -fsanitize=address,undefined -fno-sanitize-recover=all.

Fixes #99.