inet: Only try to read file contents if it exits

When the file doesn't exits we get

  Cannot read /proc/net/pnp Failed to open file '/proc/net/pnp': No such
  file or directory

in the log.

src/inet.c

@@ -2997,7 +2997,8 @@ static int get_nfs_server_ip(const char *cmdline_file, const char *pnp_file,
 		goto out;
 	}
 
-	if (!g_file_get_contents(pnp_file, &pnp, NULL, &error)) {
+	if (g_file_test(pnp_file, G_FILE_TEST_EXISTS) &&
+			!g_file_get_contents(pnp_file, &pnp, NULL, &error)) {
 		connman_error("%s: Cannot read %s %s\n", __func__,
 				pnp_file, error->message);
 		goto out;