whisper : increase tokenizer buffer (close ggml-org#1259)

Author: ggerganov

whisper.cpp

@@ -4520,7 +4520,7 @@ int whisper_full_with_state(
 
         // initial prompt
         if (!params.prompt_tokens && params.initial_prompt) {
-            prompt_tokens.resize(1024);
+            prompt_tokens.resize(2048);
             prompt_tokens.resize(whisper_tokenize(ctx, params.initial_prompt, prompt_tokens.data(), prompt_tokens.size()));
             params.prompt_tokens   = prompt_tokens.data();
             params.prompt_n_tokens = prompt_tokens.size();