matrix-net: Properly free a hashtable

We were freeing the hashtable value but we were not passing
a copy of the string, so the same memory location was being
freed multiple times.

Also clear the memory before freeing because we don't want
access token to be in memory after free.

src/matrix/matrix-net.c

@@ -330,9 +330,10 @@ queue_data (MatrixNet  *self,
 
   if (self->access_token) {
     if (!query)
-      query = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, g_free);
+      query = g_hash_table_new_full (g_str_hash, g_str_equal, g_free,
+                                     (GDestroyNotify)matrix_utils_free_buffer);
 
-    g_hash_table_replace (query, g_strdup ("access_token"), self->access_token);
+    g_hash_table_replace (query, g_strdup ("access_token"), g_strdup (self->access_token));
     soup_uri_set_query_from_form (uri, query);
   }