[MPlayer-dev-eng] Fix NULL pointer dereference when audio filter fails

[Lasse Kärkkäinen]

> can you guys paste the command lines (and possibly samples)
> required to generate such crashes?

Play a file with AC3 audio in a sample rate that your sound card does
not support (in my case 44100 Hz is the only rate supported by the sound
card and the source material is 48 kHz), with -ac hwac3. Kinda hard to
reproduce with different hardware...