PyImathFixedVArray: argument 1 value exceeds maximum object size

I assume this should be pretty easy to address? It's currently a warning but warnings often become errors in newer versions of gcc.

```
/builddir/build/BUILD/Imath-3.1.8/src/python/PyImath/PyImathFixedVArray.cpp: In member function '__ct_base ':
/builddir/build/BUILD/Imath-3.1.8/src/python/PyImath/PyImathFixedVArray.cpp:101:44: warning: argument 1 value '18446744073709551615' exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
101 | boost::shared_array > a(new std::vector[length]);
| ^
/usr/include/c++/13/new:128:26: note: in a call to allocation function 'operator new []' declared here
128 | _GLIBCXX_NODISCARD void* operator new[](std::size_t) _GLIBCXX_THROW (std::bad_alloc)
| ^
/builddir/build/BUILD/Imath-3.1.8/src/python/PyImath/PyImathFixedVArray.cpp: In member function '__ct_base ':
/builddir/build/BUILD/Imath-3.1.8/src/python/PyImath/PyImathFixedVArray.cpp:117:44: warning: argument 1 value '18446744073709551615' exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
117 | boost::shared_array > a(new std::vector[length]);
| ^
/usr/include/c++/13/new:128:26: note: in a call to allocation function 'operator new []' declared here
128 | _GLIBCXX_NODISCARD void* operator new[](std::size_t) _GLIBCXX_THROW (std::bad_alloc)
| ^
```

Inspect src/python/PyImath/PyImathFixedVArray.cpp at lines 101 and 117, where the reported vector allocation occurs, and reproduce the warning with GCC 13. Done means the affected build no longer emits the argument-1 maximum-object-size warning.

Written by the indexing model from the issue text.