Fix out-of-bounds access to FieldDims array in IOStream.cpp

[tukss]

When building with GCC 15, which introduced assertion errors for out-of-bounds accesses to std::vector, I see these errors in several of the tests:

/usr/include/c++/15/bits/stl_vector.h:1263: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](size_type) [with _Tp = int; _Alloc = std::allocator<int>; reference = int&
; size_type = long unsigned int]: Assertion '__n < this->size()' failed.   

They originate in two different parts of IOStream.cpp, where the FieldDims array was too small. This PR fixes that by changing the size accordingly.

As this is my first PR to Omega, I'd appreciate some comments because I don't know the details of the I/O framework.

[philipwjones]

Thanks for the fixes and they are indeed correct. Verified that Ctests still pass and the files look as expected. Since this is your first PR, we won't make a big deal about process, but for future PRs, please use the E3SM/Omega branch naming convention (ie starting with component name so this would be omega/fix-out-of-bounds). Also, for bug fix PRs, it's good to have the bug reported as an issue first. No worries this time and thanks again.