Null check before freeing stats memory

While doing the module cleanup, incase of module init, the stats
memory needs to be cleaned up only it is already allocated. Accessing it
without validating it is allocated or not, leads to crash.
closes-bug: #1475558

Change-Id: Iaf0d67014174506d51bd9a46671e64d463d0db71

dp-core/vr_ip_mtrie.c

@@ -1026,6 +1026,9 @@ mtrie_stats_cleanup(struct vr_rtable *rtable, bool soft_reset)
 {
     unsigned int i, stats_memory_size;
 
+    if (!mtrie_vrf_stats)
+        return;
+
     stats_memory_size = sizeof(struct vr_vrf_stats) * vr_num_cpus;
     for (i = 0; i < rtable->algo_max_vrfs; i++) {
         if (mtrie_vrf_stats[i]) {