.mov and .png open crash

When I was cross-compiling and using this library, I found that reading XMP metadata from .mov and .png files could cause crashes. Therefore, I added the following code between lines 70 and 71 in ISOBaseMedia_Support.cpp:
```
#if defined(_MSC_VER)
_ReadWriteBarrier();
#elif defined(__GNUC__) || defined(__clang__)
asm volatile("" ::: "memory");
#endif
```

And added the following code before line 232 in PNG_Support.cpp:
```
if (tempBuffer + 8 > bufferLimit) {
break;
}
```

Do you approve of my changes?
I look forward to your response. Thank you.

Start with ISOBaseMedia_Support.cpp around lines 70-71 and PNG_Support.cpp before line 232, then reproduce XMP reads for .mov and .png in the reported cross-compilation setup. Review whether the proposed memory barriers and buffer-limit check prevent the crashes without introducing platform-specific issues; done means both file types can be read without crashing.

Written by the indexing model from the issue text.