Repository: ggml-org/llama.cpp
Pull request: common: add bounds check in common_init_result::sampler to prevent segfault on failed model load

## Overview

<!-- Describe what this PR does and why. Be concise but complete -->
This fix supercedes #21049 to fix the segfault when non-existent models are loaded. It reverts the previous fix because it is no longer necessary with this more correct fix in place. In addition a regression test was added to expose the issue and showcase the fix.

_Before:_
```
./build/bin/test-init-fail
build: 8557 (43d7135f7) with GNU 13.3.0 for Linux x86_64
common_init_result: fitting params to device memory, for bugs during this step try to reproduce them with -fit off, or provide --verbose logs if the bug only occurs with -fit on
gguf_init_from_file: failed to open GGUF file '/non_existent_model.gguf' (No such file or directory)
llama_model_load: error loading model: llama_model_loader: failed to load model from /non_existent_model.gguf
llama_model_load_from_file_impl: failed to load model
llama_params_fit: encountered an error while trying to fit params to free device memory: failed to load model
llama_params_fit: fitting params to free memory took 0.00 seconds
gguf_init_from_file: failed to open GGUF file '/non_existent_model.gguf' (No such file or directory)
llama_model_load: error loading model: llama_model_loader: failed to load model from /non_existent_model.gguf
llama_model_load_from_file_impl: failed to load model
common_init_from_params: failed to load model '/non_existent_model.gguf'
Segmentation fault (core dumped)
```

_After:_
```
./build/bin/test-init-fail
build: 8557 (43d7135f7) with GNU 13.3.0 for Linux x86_64
common_init_result: fitting params to device memory, for bugs during this step try to reproduce them with -fit off, or provide --verbose logs if the bug only occurs with -fit on
gguf_init_from_file: failed to open GGUF file '/non_existent_model.gguf' (No such file or directory)
llama_model_load: error loading model: llama_model_loader: failed to load model from /non_existent_model.gguf
llama_model_load_from_file_impl: failed to load model
llama_params_fit: encountered an error while trying to fit params to free device memory: failed to load model
llama_params_fit: fitting params to free memory took 0.00 seconds
gguf_init_from_file: failed to open GGUF file '/non_existent_model.gguf' (No such file or directory)
llama_model_load: error loading model: llama_model_loader: failed to load model from /non_existent_model.gguf
llama_model_load_from_file_impl: failed to load model
main: OK
common_init_from_params: failed to load model '/non_existent_model.gguf'
```

# Requirements

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- I have read and agree with the [contributing guidelines](https://github.com/ggml-org/llama.cpp/blob/master/CONTRIBUTING.md)
- AI usage disclosure: YES AI was used to write the unit test<!-- mention: YES / NO - if yes, describe how AI was used -->

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Changed files:
filename: common/common.cpp
patch: @@ -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();
 }
 

filename: tools/completion/completion.cpp
patch: @@ -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);