Return localhost from NanostackInterface::get_ip_address()

For NetworkStack::gethostbyname() to properly validate the
IP address version, we must return a valid address from
Networkstack::get_ip_address().

features/nanostack/FEATURE_NANOSTACK/nanostack-interface/NanostackInterface.cpp

@@ -457,8 +457,10 @@ NanostackInterface * NanostackInterface::get_stack()
 
 const char * NanostackInterface::get_ip_address()
 {
-    // Unsupported
-    return NULL;
+    // Must result a valid IPv6 address
+    // For gethostbyname() to detect IP version.
+    static const char localhost[] = "::";
+    return localhost;
 }
 
 nsapi_error_t NanostackInterface::socket_open(void **handle, nsapi_protocol_t protocol)