common: add bounds check in common_init_result::sampler to prevent segfault on failed model load (#21082)

* common: add bounds check in common_init_result::sampler to prevent segfault on failed model load

* Revert a308e58

* Add regression test

* Remove regression test for init-fail sampler check

Author: mtmcp

common/common.cpp

@@ -1243,6 +1243,9 @@ llama_context * common_init_result::context() {
 }
 
 common_sampler * common_init_result::sampler(llama_seq_id seq_id) {
+    if (seq_id < 0 || seq_id >= (int) pimpl->samplers.size()) {
+        return nullptr;
+    }
     return pimpl->samplers[seq_id].get();
 }
 

tools/completion/completion.cpp

@@ -146,19 +146,13 @@ int main(int argc, char ** argv) {
 
     ctx   = llama_init->context();
     model = llama_init->model();
+    smpl  = llama_init->sampler(0);
 
     if (ctx == NULL) {
         LOG_ERR("%s: error: unable to create context\n", __func__);
         return 1;
     }
 
-    if (model == NULL) {
-        LOG_ERR("%s: error: unable to load model\n", __func__);
-        return 1;
-    }
-
-    smpl = llama_init->sampler(0);
-
     llama_memory_t mem = llama_get_memory(ctx);
     const llama_vocab * vocab = llama_model_get_vocab(model);