whisper : fix condition for providing past prompt (critical)

This bug has been present since v1.1.0.

Effectively, the past transcribed text wasn't being used for following
transcriptions, which likely significantly reduces the transcription
quality.

Likely related to ggerganov#419

whisper.cpp

@@ -3449,7 +3449,7 @@ int whisper_full(
                 prompt.clear();
 
                 // if we have already generated some text, use it as a prompt to condition the next generation
-                if (!prompt_past.empty() && t_cur > 0.5f) {
+                if (!prompt_past.empty() && t_cur < 0.5f) {
                     int n_take = std::min(std::min(params.n_max_text_ctx, whisper_n_text_ctx(ctx)/2), int(prompt_past.size()));
 
                     prompt = { whisper_token_prev(ctx) };