server: free sampling contexts on exit

This cleans up last leak found by the address sanitizer.

examples/server/server.cpp

@@ -671,6 +671,12 @@ struct server_context {
             model = nullptr;
         }
 
+	// Clear any sampling context
+	for (server_slot & slot : slots) {
+	    if (slot.ctx_sampling != nullptr)
+	        llama_sampling_free(slot.ctx_sampling);
+	}
+
         llama_batch_free(batch);
     }