[3.13] gh-123229: Fix valgrind warning by initializing the f-string buffers to 0 in the tokenizer (GH-123263) (#123264)

gh-123229: Fix valgrind warning by initializing the f-string buffers to 0 in the tokenizer (GH-123263)
(cherry picked from commit adc5190014efcf7b7a4c5dfc9998faa8345527ed)

Signed-off-by: Pablo Galindo <pablogsal@gmail.com>
Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>