Make backend's top_p sampler inclusive

In addition to match the algorithm proposed in the original
[paper](https://arxiv.org/abs/1904.09751), this resolves the edge-case
where `max_p is > top_p` for a single logit, where the mask would
otherwise be empty (and we thus sample from the whole vocabulary with
equal likelihood)

tests/test-backend-sampler.cpp

@@ -512,6 +512,7 @@ static void test_backend_top_p_sampling(const char * model_path) {
         }
     }
     GGML_ASSERT(filtered_logits.size() < (size_t) test_ctx.n_vocab);
+    GGML_ASSERT(filtered_logits.size() > 0);
 
     // Sample using CPU sampler for verification to inspect they are reasonable
     struct llama_sampler_chain_params chain_params = llama_sampler_chain_default_params();