fix resource leak of utsname in error path found by coverity

src/lxc/confile.c

@@ -1344,6 +1344,7 @@ static int config_utsname(const char *key, const char *value,
 	if (strlen(value) >= sizeof(utsname->nodename)) {
 		ERROR("node name '%s' is too long",
 			      utsname->nodename);
+		free(utsname);
 		return -1;
 	}