server : check draft context creation error (ggml-org#24922)

Author: Kononnable

tools/server/server-context.cpp

@@ -89,7 +89,9 @@ struct server_batch {
     }
 
     ~server_batch() {
-        llama_batch_free(batch);
+        if (batch.token != nullptr) {
+            llama_batch_free(batch);
+        }
     }
 
     void init(int32_t n_tokens_alloc) {
@@ -1215,6 +1217,10 @@ struct server_context_impl {
             cparams.ctx_other = ctx_tgt;
 
             ctx_dft.reset(llama_init_from_model(model_dft.get(), cparams));
+            if (ctx_dft == nullptr) {
+                SRV_ERR("%s", "failed to create draft context\n");
+                return false;
+            }
 
             params_base.speculative.draft.ctx_tgt = ctx_tgt;
             params_base.speculative.draft.ctx_dft = ctx_dft.get();