BUG/MINOR: h3: fix uninitialized value in h3_req_headers_send()

BUG/MINOR: h3: fix uninitialized value in h3_req_headers_send()

Due to the introduction of smallbuf usage for HTTP/3 headers emission,
ret variable may be used uninitialized if buffer allocation fails due to
not enough room in QUIC connection window.

Fix this by setting ret value to 0.

Function variable declaration are also adjusted so that the pattern is
similar to h3_resp_headers_send(). Finally, outbuf buffer is also
removed as it is now unused.

No need to backport.