remove unnecessary NULL checks before free()

[RokerHRO]

I found several unnecessary checks for NULL before call of free().

I removed them.

I don't remove the check in code that sets the pointer to NULL after the free:

if(p)
{
    free(p);
    p = NULL;
}

Additionally I found a questionable macro FREE_IF in main.c which seems to be a shortcut to free (with unnecessary NULL check) a member of struct screen. I renamed it to FREE_SCREEN_MEMBER to reflect its purpose better, but it is used only twice, so I'd favor to remove this macro completely.

[bk138]

Thank you very much!