rtmpproto: increase APP_MAX_LENGTH

Fixes second part of Ticket2292

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit da8ef5ac2f1b14f4fe34db72af5d5f8b3c8e2ff9)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>