chore: Fix memory leak in callbackRetText function

diff --git a/callback.go b/callback.go
index b794bcd..84612b4 100644
--- a/callback.go
+++ b/callback.go

@@ -345,7 +345,9 @@
 	if v.Type().Kind() != reflect.String {
 		return fmt.Errorf("cannot convert %s to TEXT", v.Type())
 	}
-	C._sqlite3_result_text(ctx, C.CString(v.Interface().(string)))
+	cstr := C.CString(v.Interface().(string))
+	defer C.free(unsafe.Pointer(cstr))
+	C._sqlite3_result_text(ctx, cstr)
 	return nil
 }