sampling : add clamping to backend dist sampler

This commit adds clamping to the backend distribution sampler to avoid
the case where idxf values are all zero. If this happens then we will
incorrectly create an out of bounds idx value which will cause a crash.

This can be reproduced by explicitly setting idxf to zero:
```c++
    idxf = ggml_scale(ctx, idxf, 0.0f);
```

src/llama-sampler.cpp

@@ -1180,6 +1180,9 @@ static void llama_sampler_dist_backend_apply(
     struct ggml_tensor * idxf = ggml_sum(ctx, mask);
     ggml_set_name(idxf, "dist_index_f32");
 
+    // Clamp to prevent out-of-bounds access when computing the index.
+    idxf = ggml_clamp(ctx, idxf, 1.0f, mask->ne[0]);
+
     // Use ggml_scale_bias to scale the index value by -1 and then add the size
     // of the mask to that value so we get the correct index ((-1 * idxf) + n).
     struct ggml_tensor * idx = ggml_cast(ctx, ggml_scale_bias(ctx, idxf, -1.0f, mask->ne[0]), GGML_TYPE_I32);