adios2_inquire_group_variables returns uninitialized pointer

[eschnett]

The function adios2_inquire_group_variables returns an uninitialized pointer (via its variables argument) when the group is empty.

These lines in its implementation cause the problem:

        *size = names.size();
        if (*size == 0)
            return adios2_error_none;

When names is empty, the function returns adios2_error_none, but *variables has not been assigned a value.

I think that the code should *variables = nullptr in this case.

Alternatively, the if statement could be removed. The code below should handle the empty case correctly.