Introducing basic_free_stack_if_not_null

[anutosh491]

Addressing #2614 (comment)

We have been using the basic_free_stack out of symengine's C wrapper untill now. But before freeing out variables we need to check if they have been stack allocated previously. Hence this PR aims to replace every basic_free_stack call with

    if (var != NULL) {
        basic_free_stack(var);
    }

[anutosh491]

I tried approach 1 & 2 as mentioned here (#2614 (comment)) but haven't been able to address the example in consideration fully. cc @Thirumalai-Shaktivel could you try helping me out here ?

[certik]

@anutosh491 what is the status here --- can you describe what the problem is?

Are you trying to address this comment: #2614 (comment) ?