Rename the suffix _get_at to the _safe_get suffix

(Better & clearer naming of the function)

tests/test-mbptree.c

@@ -155,10 +155,10 @@ static void test1(void)
   assert (btree_size(b) == 0);
   assert (btree_empty_p(b));
 
-  int *p = btree_get_at(b, 1000);
+  int *p = btree_safe_get(b, 1000);
   assert(p != NULL && *p == 0);
   *p = 23;
-  p = btree_get_at(b, 1000);
+  p = btree_safe_get(b, 1000);
   assert(p != NULL && *p == 23);
   
   btree_clear(b);