Fix build breakage

test/core/client_config/lb_policies_test.c

@@ -616,6 +616,8 @@ static void verify_rebirth_round_robin(const servers_fixture *f,
   expected_connection_sequence = gpr_malloc(sizeof(int) * expected_seq_length);
   seen_elements = gpr_malloc(sizeof(int) * expected_seq_length);
 
+  unique_seq_last_idx = ~(size_t)0;
+
   memset(seen_elements, 0, sizeof(uint8_t) * expected_seq_length);
   for (i = 0; i < num_iters; i++) {
     if (actual_connection_sequence[i] < 0 ||
@@ -638,6 +640,8 @@ static void verify_rebirth_round_robin(const servers_fixture *f,
     GPR_ASSERT(seen_elements[j] != 0);
   }
 
+  GPR_ASSERT(unique_seq_last_idx != ~(size_t)0);
+
   unique_seq_first_idx = (unique_seq_last_idx - expected_seq_length + 1);
   memcpy(expected_connection_sequence,
          actual_connection_sequence + unique_seq_first_idx,