[SSSD] [PATCH] Avoid accessing half-deallocated memory when using talloc_zfree macro

[Eugene Indenbom]

The current implementation of talloc_zfree macro has a substantial 
deficiency: it deallocates memory first and then clears a pointer.

Clearing pointer after deallocation completes could lead to bad side 
effects when callbacks are called during object destruction: the 
callback called from destructor could dereference half-destroyed object 
referenced by the yet-not-cleared pointer.

The above could lead to disastrous effects including memory double free.

The patch attached fixes the issues by using correct free sequence: 
clear pointer first, then deallocate memory block.

Eugene