Avoid passing empty string to nsock_pool_set_device

Nsock checks for NULL, but not for empty string. Rather than making
Nsock decide, each app will decide whether to call
nsock_pool_set_device, with all currently skipping the call if device is
an empty string.

FPEngine.cc

@@ -139,6 +139,7 @@ void FPNetworkControl::init(const char *ifname, devtype iftype) {
   nmap_set_nsock_logger();
   nmap_adjust_loglevel(o.packetTrace());
 
+  if (*o.device)
     nsock_pool_set_device(nsp, o.device);
 
   if (o.proxy_chain)

nmap_dns.cc

@@ -1381,6 +1381,7 @@ static void nmap_mass_dns_core(DNS::Request *requests, int num_requests) {
     nmap_set_nsock_logger();
     nmap_adjust_loglevel(o.packetTrace());
 
+    if (*o.device)
       nsock_pool_set_device(dnspool, o.device);
 
     if (o.proxy_chain)

nping/ProbeMode.cc

@@ -97,7 +97,9 @@ int ProbeMode::init_nsock(){
       /* Create a new nsock pool */
       if ((nsp = nsock_pool_new(NULL)) == NULL)
         nping_fatal(QT_3, "Failed to create new pool.  QUITTING.\n");
-      nsock_pool_set_device(nsp, o.getDevice());
+      const char *device = o.getDevice();
+      if (*device)
+        nsock_pool_set_device(nsp, device);
 
       /* Allow broadcast addresses */
       nsock_pool_set_broadcast(nsp, 1);

nse_nsock.cc

@@ -86,6 +86,7 @@ static nsock_pool new_pool (lua_State *L)
   nmap_set_nsock_logger();
   nmap_adjust_loglevel(o.scriptTrace());
 
+  if (*o.device)
     nsock_pool_set_device(nsp, o.device);
 
   if (o.proxy_chain)

service_scan.cc

@@ -2817,6 +2817,7 @@ int service_scan(std::vector<Target *> &Targets) {
   nmap_set_nsock_logger();
   nmap_adjust_loglevel(o.versionTrace());
 
+  if (*o.device)
     nsock_pool_set_device(nsp, o.device);
 
   if (o.proxy_chain) {