netmap: fix coverity warning 1362789

netmap: fix coverity warning 1362789

** CID 1362789:  Null pointer dereferences  (FORWARD_NULL)
/src/runmode-netmap.c: 247 in ParseNetmapConfig()

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*** CID 1362789:  Null pointer dereferences  (FORWARD_NULL)
/src/runmode-netmap.c: 247 in ParseNetmapConfig()
241         strlcpy(aconf->iface_name, iface_name, sizeof(aconf->iface_name));
242         SC_ATOMIC_INIT(aconf->ref);
243         (void) SC_ATOMIC_ADD(aconf->ref, 1);
244
245         /* Find initial node */
246         netmap_node = ConfGetNode("netmap");
>>>     CID 1362789:  Null pointer dereferences  (FORWARD_NULL)
>>>     Comparing "netmap_node" to null implies that "netmap_node" might be null.
247         if (netmap_node == NULL) {
248             SCLogInfo("Unable to find netmap config using default value");
249         } else {
250             if_root = ConfFindDeviceConfig(netmap_node, aconf->iface_name);
251             if_default = ConfFindDeviceConfig(netmap_node, "default");
252         }