fix double free

Author: am17an

tools/server/server-context.cpp

@@ -711,18 +711,18 @@ struct server_context_impl {
     void destroy() {
         llama_init.reset();
 
-        ctx = nullptr;
-        model = nullptr;
-
-        mtmd_free(mctx);
-        mctx = nullptr;
-
         for (server_slot & slot : slots) {
             if (slot.can_speculate()) {
                 slot.spec.reset();
             }
         }
 
+        ctx = nullptr;
+        model = nullptr;
+
+        mtmd_free(mctx);
+        mctx = nullptr;
+
         llama_batch_free(batch);
     }