Bug 1694263: Fix thread private variable initialization
    
    Fix description:
    	The initialization of the thread private variable was buggy.
    	The private variable required an allocated struct (per thread).
    	The struct is allocated on demand (during get/set) when the
    	retrieved (PR_GetThreadPrivate) struct is NULL.
    
    Signed-off-by: Thierry Bordaz <tbordaz@redhat.com>