quic: Check we have some data to start decoding quic image

All paths already pass some data to quic_decode_begin but for the
test check it, it's not that expensive test.
Checking for not 0 is enough, all other words will potentially be
read calling more_io_words but we need one to avoid a potential
initial buffer overflow or deferencing an invalid pointer.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Acked-by: Uri Lublin <uril@redhat.com>