Prevent corruption of server.executable after DEBUG RESTART.

Doing the following ended with a broken server.executable:

1. Start Redis with src/redis-server
2. Send CONFIG SET DIR /tmp/
3. Send DEBUG RESTART

At this point we called execve with an argv[0] that is no longer related
to the new path. So after the restart the absolute path of the
executable is recomputed in the wrong way. With this fix we pass the
absolute path already computed as argv[0].

src/server.c

@@ -1583,6 +1583,8 @@ int restartServer(int flags, mstime_t delay) {
 
     /* Execute the server with the original command line. */
     if (delay) usleep(delay*1000);
+    zfree(server.exec_argv[0]);
+    server.exec_argv[0] = zstrdup(server.executable);
     execve(server.executable,server.exec_argv,environ);
 
     /* If an error occurred here, there is nothing we can do, but exit. */