ffserver: Check chunk size

      Fixes out of array access

Fixes: poc_ffserver.py
Found-by: Paul Cher <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a5d25faa3f4b18dac737fdb35d0dd68eb0dc2156)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>