ffplay: dynamically allocate filename buffer

filename was set to an arbitrary 1024 characters. ffplay would thus be unable to
play files whose name exceeds that arbitrary threshold.

This patch dynamically allocates and frees the filename buffer to remove such
limitations.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>
