Fix warnings found by cppcheck (ARMmbed#2605)

- ns_socket.c: Uninitialized variable: tcp_info
- mac_security_mib.c: Possible null pointer dereference: device_info

Author: Arto Kinnunen

source/Core/ns_socket.c

@@ -1119,7 +1119,7 @@ int16_t socket_buffer_sendmsg(int8_t sid, buffer_t *buf, const struct ns_msghdr
 
 #ifndef NO_TCP
     if (socket_ptr->type == SOCKET_TYPE_STREAM) {
-        tcp_session_t *tcp_info = tcp_info(inet_pcb);
+        tcp_session_t *tcp_info = inet_pcb->session;
         if (!tcp_info) {
             tr_warn("No TCP session for cur Socket");
             ret_val = -3;

source/MAC/IEEE802_15_4/mac_security_mib.c

@@ -600,7 +600,7 @@ void mac_sec_mib_key_device_frame_counter_set(mlme_key_descriptor_t *key_descpri
     if (key_descpription_table->KeyFrameCounterPerKey) {
         uint32_t *counter_ptr = key_descpription_table->KeyDeviceFrameCounterList + attribute_index;
         *counter_ptr = frame_counter;
-    } else {
+    } else if (device_info) {
         device_info->FrameCounter = frame_counter;
     }
 }