tools: block using lxc-execute without config file

Moving away from internal symbols we can't do hacks like we currently do in
lxc-start and call internal functions like lxc_conf_init(). This is unsafe
anyway. Instead, we should simply error out if the user didn't give us a
configuration file to use. lxc-start refuses to start in that case already.

Relates to discussion in lxc/go-lxc#96 (comment) .
Closes #2023.

Reported-by: Felix Abecassis <fabecassis@nvidia.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/tools/lxc_execute.c

@@ -148,6 +148,12 @@ int main(int argc, char *argv[])
 		}
 	}
 
+	if (!c->lxc_conf) {
+		fprintf(stderr, "Executing a container with no configuration file may crash the host\n");
+		lxc_container_put(c);
+		exit(EXIT_FAILURE);
+	}
+
 	ret = lxc_config_define_load(&defines, c->lxc_conf);
 	if (ret) {
 		lxc_container_put(c);