fix: fix bug where multiple fields' structures can only read the first field with mat73

[usopp333]

s.a = [1, 2, 3,4]; s.b=[2,3,4,5];
save('s.mat', 's');

mat_t* matfp = Mat_Open(filename, MAT_ACC_RDONLY);
matvar_t* matvar = Mat_VarReadNext(matfp);
unsigned fields_num = Mat_VarGetNumberOfFields(matvar); // fields_num == 1

• mat73: s.a = [1, 2, 3,4]; s.b=[2,3,4,5]; save('s.mat', 's'); only read s.a;
• mat5: no bug
• fix bug by Cursor-AI
• Update H5Literate return value handling in Mat_H5ReadGroupInfo and Mat_H5ReadGroupInfoIterate functions