Fix memory leak in callbackRetText function

[hionay]

This PR fixes the memory leak in the callbackRetText function by freeing the memory allocated by the code:

C.CString(v.Interface().(string))

[otoolep]

Real LLM vibes with that last review comment.

[hionay]

Hi @mattn, how can I help in getting this merged?

[charlievieth]

@mattn / @rittneje this is a pretty straightforward (and correct) fix for an obvious bug - could either of you take a look at this PR?

[hionay]

@mattn Have you had a chance to look at this PR?

[mattn]

thank you. please wait

[mattn]

Thank you

[mattn]

I revert this PR. This break original behavior.

[mattn]

Original code have no problem.

[mattn]

void _sqlite3_result_text(sqlite3_context* ctx, const char* s) {
  sqlite3_result_text(ctx, s, -1, &free);
}

This s is free-ed correctly.