booru/imagefetcher: Assign new SockInfo after setting up event callback

This caused a rare issue were files would never get downloaded.
It was very  easy to trigger when downloading multiple files
at the same time either when the cache size is set to something
greater than zero or when saving images.

src/booru/imagefetcher.cc

@@ -6,17 +6,20 @@ int ImageFetcher::socket_cb(CURL*, curl_socket_t s, int action, void *userp, voi
     ImageFetcher *self = static_cast<ImageFetcher*>(userp);
     SockInfo *fdp = static_cast<SockInfo*>(sockp);
 
-    if (action == CURL_POLL_REMOVE && fdp)
+    if (action == CURL_POLL_REMOVE)
     {
-        delete fdp;
+        if (fdp)
+            delete fdp;
     }
     else
     {
+        bool need_assign = false;
+
         if (!fdp)
         {
+            need_assign = true;
             fdp = new SockInfo();
             fdp->chan = Glib::IOChannel::create_from_fd(s);
-            curl_multi_assign(self->m_MultiHandle, s, fdp);
         }
 
         Glib::IOCondition kind;
@@ -31,6 +34,9 @@ int ImageFetcher::socket_cb(CURL*, curl_socket_t s, int action, void *userp, voi
 
         fdp->conn = source->connect(sigc::bind<0>(sigc::mem_fun(self, &ImageFetcher::event_cb), s));
         source->attach(self->m_MainContext);
+
+        if (need_assign)
+            curl_multi_assign(self->m_MultiHandle, s, fdp);
     }
 
     return 0;