tests/gnrc_ipv6_nib_6ln: Refactored code

Improved readability of if statements. This issue was pointed out by cppcheck.

tests/gnrc_ipv6_nib_6ln/main.c

@@ -1075,11 +1075,9 @@ static void test_handle_pkt__rtr_adv__success(uint8_t rtr_adv_flags,
             TEST_ASSERT((_PIO_PFX_LTIME / MS_PER_SEC) < prefix.pref_until);
         }
     }
-    if (!pio) {
-        if (!pio) {
-            TEST_ASSERT_EQUAL_INT(exp_addr_count,
-                                  _netif_addr_count(_mock_netif));
-        }
+    else {
+        TEST_ASSERT_EQUAL_INT(exp_addr_count,
+                              _netif_addr_count(_mock_netif));
         TEST_ASSERT_MESSAGE(!gnrc_ipv6_nib_pl_iter(0, &state, &prefix),
                             "There is an unexpected prefix list entry");
     }