posix: pthread_exit: check for null before dereferencing

In a primitive SYS_SLIST_FOR_EACH_NODE check for null was
after dereferencing. Place check for null of the "thread_spec_data"
before its dereferencing.

Found as a coding guideline violation (MISRA R4.1) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>