rtl-allocator.c: Dereference null return value error spotted by Coverity

[Ryan Long]

CID 1444139: Dereference null return value in rtems_rtl_alloc_hook().

   1. returned_null: rtems_rtl_lock returns NULL (checked 23 out of 28 times). [show details]
   2. var_assigned: Assigning: rtl = NULL return value from rtems_rtl_lock.
164  rtems_rtl_data*     rtl = rtems_rtl_lock ();
   CID 1444139 (#1 of 1): Dereference null return value (NULL_RETURNS)3. dereference: Dereferencing rtl, which is known to be NULL.
165  rtems_rtl_allocator previous = rtl->allocator.allocator;