* c-common,c (get_atomic_generic_size):  Handle pointer to 0 size.

Index: c-family/c-common.c
===================================================================
*** c-family/c-common.c	(revision 211758)
--- c-family/c-common.c	(working copy)
*************** get_atomic_generic_size (location_t loc,
*** 10462,10467 ****
--- 10462,10468 ----
      {
        int size;
        tree type = TREE_TYPE ((*params)[x]);
+       tree type_size;
        /* __atomic_compare_exchange has a bool in the 4th position, skip it.  */
        if (n_param == 6 && x == 3)
          continue;
*************** get_atomic_generic_size (location_t loc,
*** 10471,10477 ****
  		    function);
  	  return 0;
  	}
!       size = tree_to_uhwi (TYPE_SIZE_UNIT (TREE_TYPE (type)));
        if (size != size_0)
  	{
  	  error_at (loc, "size mismatch in argument %d of %qE", x + 1,
--- 10472,10480 ----
  		    function);
  	  return 0;
  	}
! 
!       type_size = TYPE_SIZE_UNIT (TREE_TYPE (type));
!       size = type_size ? tree_to_uhwi (type_size) : 0;
        if (size != size_0)
  	{
  	  error_at (loc, "size mismatch in argument %d of %qE", x + 1,