lavd/pulse_audio_enc: fix array compared against 0

fixes CID 1113222

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavd/pulse_audio_enc: more stream validation restrictive

So far pulse device picked up first audio stream and ignored others.
It is required to provide exactly one audio stream now.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavd/fbdev_enc: more stream validation restrictive

So far fbdev_enc device picked up first video stream and ignored others.
It is required to provide exactly one video stream now.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>